Control of the resources or territory wasn’t magically delivered to the people with equity. The colonial infrastructure of control was turned over to local friendly interests and their successors. (Through revolutions, coups, etc)
Handwavy, vague "post-colonial" whatever isn't a reason, it's an excuse.
Plus, if there’s a popular revolt, they are usually motivated by religious, ethnic or ideological factors. Status quo is the interest of the big powers. That tends to bring direct or advisory intervention by western military forces.
I’d suggest spending a few minutes googling, you’ll learn alot.
These are as a category unstable political situations and it is vanishingly few who manage to develop their way out of it. More common is descent into further chaos, with major powers standing by to ensure that it does not disrupt resource extraction.