To be fair, the US and Israel have generally branded anyone who refuses to play ball with them a terrorist group (often with good reason), which means that Arab states are leery of providing support for Palestinian institutions outside of US-approved channels (due to sanctions and the like). They're also unhappy about using a US framework, though, since that gives them little say in what happens and looks bad on the Arab street. Support for Hamas-affiliated social service organizations is generally off-limits, which means that real help for Gaza (which needs it far more than the West Bank) is tough to manage outside of UNRWA. Meanwhile, help for the West Bank institutions is tightly controlled by Israel, especially when it comes to security arrangements. Growing portions of PA security forces have been trained under US auspices in Jordan, and they are by far the most professional of the PA forces (and most willing to coordinate with the IDF); can you imagine Israel allowing Saudi or UAE or Egypt (or Syria) to do something similar? Hell, given the chronic corruption and poor training in most of their own security forces, I can't imagine that they would turn out recruits worth half as much as the US-trained forces.

Honestly I don't think money is the problem; the real issue is the corruption endemic to the PA's political system, which siphons much of the aid and stifles real reform. I doubt the Arab states can do much about this.