And why is that?
Where did the literal billions of dollars go and what do they have to show for it?

It was stolen/wasted by Hamas.

But again, that doesn't resolve the problem. Hamas isn't the Palestinian people. They're the dictatorship that is ruling by force a small portion (less than 20%) of the Palestinian people. The fact that Hamas is terrible, that Hamas has wasted billions of dollars on guns instead of butter, does not invalidate the legitimate demands for autonomy and self-determination of the millions of Palestinians both inside and outside Gaza.

You can't say that because Hamas is evil, the Palestinians don't have a legitimate claim to self-determination. Because the Palestinian people don't control Hamas. That's why conflicts involving non-state actors are so horrifically complex to resolve. Hamas is doing horrific things, but those horrific things don't - and can't - be attributed to the Palestinian people as a whole, because they have no role whatsoever in the governance or decision making of Hamas.