Most people don’t want to deal with the consequences. The damage to their families, the possibility of being killed themselves either by the cops or death penalty, the lies about them the press and political opponents would make up about them, everyone has some skeletons they wouldn’t want revealed, and the fact they’d be silenced and have no way to make a real case for why they did what they did. At best a lifetime in prison because, even if you're right, they’re gonna make an example of you because we can’t just have people killing presidents, y’know. I think also that they might realize that it’s probably cutting a head off the Hydra, some other PoS would just step up and continue the shitshow.

IOW you’d have to have someone with nothing to lose and the willingness to sacrifice themselves for such a deed, and the skills to pull it off.

Generally the kind of person willing to kill a president isn’t going to be sane, and if someone is sane and has the same idea they understand the price and aren’t going to want to do it. That, and things aren’t dire enough for a sane person to make the leap to do such a thing.