This scenario seems rather plausible; I don't know why they didn't think of this IOTL.
I would give a shot and suggest it was because of the state of constant civil war in late Lagid Egypt, a complex situation that Rome didn't really need with his own civil wars ongoing.
It was simply easier to clientelize Egypt, without the troubles that would come with ruling it.
As getting rid of the civil unrest in late Lagid Egypt is probably going to implies an earlier PoD which is likely to butterfly away Roman influence, at least partially; your best chance would be to deal with Late Republican civil wars (which is another issue on its own right) at least time to accept the will and having Rome experiencing buyer's remorse with the shitstorm it inherited.