We know about instances when it was tried, and usually failed, but we also know about the rest of history when as far as we know it wasn’t tried or even contemplated for the exact reasons the OP stated.

> The pre-french kings paid off The tribes who eventually became the Normans.

Probably the worst decision any French leader, possibly any European leader, ever made. It could be argued that this lead to at least 600 years of pain until the Normans aristocracy (now English) gave up trying to take France.