Because I think positive well-being is good, rather than neutral. (This strikes me as more plausible in principle, and also when reflecting on specific cases, e.g. a fantastic life involving a few minor harms.)

Here is a classic piece on the subject by Toby. Brief quote from one line of argument:

Absolute NU is completely indifferent to happiness (over and above any merely instrumental effects it has on reducing suffering). Suppose there were a world that consisted of a thriving utopia, filled with love, excitement, and joy of the highest degree, with no trace of suffering. One day this world is at threat of losing almost all of its happiness. If this threat comes to pass, almost all the happiness of this world will be extinguished. To borrow from Parfit's memorable example, they will be reduced to a state where their only mild pleasures will be listening to muzak and eating potatoes. You alone have the power to decide whether this threat comes to pass. As an Absolute Negative Utilitarian, you are indifferent between these outcomes, so you decide arbitrarily to have it lose almost all of its overflowing happiness and be reduced to the world of muzak and potatoes.
 

I feel that strong negative utilitarianism (we should only consider disutility) is just a non-starter. It doesn't match any of my moral intuitions. 

But a weaker negative utilitarianism is a powerful and potentially valid position. These are my views:

1. Good things are actually good. Pleasure is usually good. Laughing/ smiling/ dancing/ sex/ rollercoasters/ MDMA trips are actually really good.
2. Bad things might be a bit worse than good things are good (at least our intuitions might be skewed). But a rudimentary thought experiment can calibrate your scales here - I don't see how you can end up at a strong negative position. 
3. There is a hedonic treadmill/ normalisation effect, making 'normality' into suffering, but this is not always the case - some hedonistic pleasure can give you a warm glow for a long time, and make you genuinely happier in the long term.
4. We probably have a weak bias towards pleasure and suffering in our lives being more balanced than they are, and towards believing that our lives are more positive than they actually are. E.g. someone can say that his life is 6/10 on a happiness scale, but he could be totally wrong, and his life could actually not be worth living. 
5. This bias is unlikely to be so strong that even the happiest-seeming people actually have net-negative lives. So there are almost definitely some net-positive lives in the world.
6. Lots of human lives are net-negative. My median estimate would be 25% (around 5/10 on a happiness scale), but I have very wide error margins. 
7. This is just a rounding error compared to animal lives, which are more likely to be net-negative, therefore the world is likely to be net-negative, even from a standard utilitarian position. 
8. Even under very weak negative utilitarianism, the 'benevolent world-exploder' argument may be valid from a near-termist view, and is compatible with fear of s-risks. But, even to a moderate weak negative utilitarian, it can be countered with an optimistic long-termism where we may be able to both eradicate suffering and normalise increasingly wondrous states of pleasure.

I've been attracted to this idea my whole adult life. However:

1. an actual attempt to pursue it would probably have quite awful consequences instead of the good ones imagined (simplest case possible to realise: me ending my own suffering would create suffering for my close ones) Killing other people - there's no magic annihilation button, so that's probably not going to end well either. Perhaps something like legalising euthanasia could actually successfully reduce suffering rather than accidentally increase it.
2. as the previous point may hint already, I don't think this philosophy is a very healthy one to hold - or rather, I believe it's a result of a mind troubled with suffering. So it's not that it changed my mind, but rather it was something I naturally looked for and arrived at thanks to being depressed - and I didn't enjoy the journey very much.

Ishaan -- I can imagine some potentially persuasive arguments that negative utilitarianism might describe the situation for many wild animal species, and perhaps for many humans in prehistory.

However, our species has been extraordinarily successful at re-engineering our environments, creating our own eco-niches, and inventing technologies that maximize positive well-being and minimize suffering. The result is that, according to all the research I've seen on happiness, subjective well-being, and flourishing, most humans in the modern world are well above 'neutral' in terms of utility. 

So the central claims of negative utilitarianism -- which we could caricature/summarize as 'life is suffering' and 'happiness is irrelevant' -- simply aren't true, empirically, for most modern humans. 

Another way to frame this is to ask real people whether they'd be content to accept a painless suicide. The vast majority will say no. Why do we think that if we aggregate this at the species level that we'd be content to accept a painless mass extinction event?

On a more personal note, as a psychology professor, I'm deeply concerned that writers such as Perry and Benatar can undermine the mental health of young adults who take philosophical questions seriously. I think their writings are basically 'information hazards' for those prone to dysthymia, depression, or psychosis. So, I think their ideas are empirically false, theoretically incoherent, and psychologically dangerous to many vulnerable people. 

I'm a "gun-to-my-head" negative utilitarian -- that is, if I'm pressed, or in case of situations and scenarios where my ordinary pragmatic parsing falls apart, if Omega asks me, so to speak, that's what I'd recommend. The alternatives are just too ghastly to contemplate. Here I think most of us are simply extremely deluded about how truly bad bad experiences can be; if we had any glimpse of a shadow of a vague idea of just how horrible extreme suffering can be, we'd all be frantically flailing about for the infamous "off button" on reality... On this topic, see: https://qualiacomputing.com/2019/08/10/logarithmic-scales-of-pleasure-and-pain-rating-ranking-and-comparing-peak-experiences-suggest-the-existence-of-long-tails-for-bliss-and-suffering/

There is a considerable body of considered thought on this area, and instead of waxing abstract about utilitarian calculus in philosophy 101 terms, I suggest consulting the work of suffering-focused ethicists, such as David Pearce, Magnus Vinding, Jonathan Leighton, Brian Tomasik, etc. The good folks at Qualia Research Institute are also trying to figure out the ground facts about this all. The nature of valence is an open question, and how any sort of utilitarian ethics will play out depends on the actual details of what value actually is, as an objective feature of reality.

I personally have found negative utilitarianism an oddly cheery ethics. After all, even the faintest hint of any despair or horror at existence is, ceteris paribus, not something that negative utilitarianism would recommend! (Negative utilitarianism might indeed be a self-limiting meme, in that in many cases the prudent negative utilitarian choice is to adopt some more psychologically adaptive explicit ethical system, and there is a strong case for not proselytizing, given the psychological harm these ideas can do at least in unsophisticated form.)