Why are NBA drafts so thin

In the NFL, teams can pretty much be assured they'll get a quality starter in the first round and maybe even into the second.

MLB is more of a crap shoot, since prospects may take years to get to the majors.

Don't know about hockey.

But in the NBA, it seems like if a team doesn't get a top 5 pick, or even a top 3-4 pick, the chances of a drafting an all-star caliber player plummet. If you look at the last few all-star games and see where the all-stars were drafted, very few were drafted outside the top 10 if not the top 5.

So here is my questions, with 32 D1 divisions (averaging over 10 teams per division) in the NCAA + overseas players from Europe, S. America, Australia, etc, why is the NBA draft so thin? Why isn't there more talent coming out of those places, and is there anything that can be done to deepen the NBA draft?