I agree with you that it would be convenient for everyone if the West Bank went to Jordan, Gaza went to Egypt, and the residents of those areas integrated nicely into those countries, and everyone lived happily ever after. But, I don't think that's a realistic possibility. It's akin to hoping that Quebec decides the French language was a bad idea, learning English, and lobbying to make Canada a unilingual English speaking country. Sure, it would make it easier for everyone, but never gonna happen.
The Palestinians don't want to be part of Jordan or Egypt, and Jordan and Egypt most definitely do not want the Palestinians.
And, of course, the "one state" solution does not work since the Palestinian population is a demographic problem for Israel, if Israel were to formally annex the West Bank, it could not remain a state whose primary character is to be the homeland of Jews while also remaining democratic since the majority of the population would soon be Arab Muslims.
Neither is the current status quo a reasonable long term situation. While a better solution has not yet presented itself, keeping the Palestinians an essentially stateless people forever is not reasonable and leads to discontent and violence.
A separate Palestinian state is therefore the only realistic solution that has a chance at long term stability.