The money donated to Palestine was mostly used to save people from the many crisises and famines that the Palestinians have endured. A lot of it went to rebuilding the education system, providing health care, and social services in a war torn, open air prison. Sure, some bit of it went to building missiles and weapons, but that doesn’t mean that they haven’t tried to “build up” Palestine.
This situation is a bit too complicated to solve by just throwing money at it, anyways. Even if Hamas didn’t spend one penny of Palestinian aid on weapons, they would still be in a pretty similar situation.