1. Tall buildings are often cordoned off
2. Suicide is incredibly hard to perform because of instinctual survival mechanisms, and because of love for one's family, friends etc.
3. There is always a chance of rescue, no matter how slim, and some people, like me, cannot afford to risk becoming paralysed with an even worse quality of life
4. Just because someone kills themselves does not mean all the suffering in their lives is negated. In fact, none of it is. In the immortal words of E.M. Cioran: "It is not worth the bother of killing yourself, since you always kill yourself too late".
The fact that suicide does not mean suffering is cancelled is one of the many things antinatalism is based upon - because people should not be forced to suffer just because happy people exist. What would those happy people be deprived of had they not existed? Nothing, because non-existent people cannot be deprived of anything. What are depressed people deprived of? A lot.
Eat this, Bryan Caplan.
EDIT: Super Duper Extra Bonus Reason: Suicide is often painful, which means you have to pay your way in suffering to actually get out of this horrible place. Which isn't exactly 'Free Disposal', is it, Bry-Bry? Was that in your plan, Caplan? Sorry, can't help myself.