It's rare because they aren't getting the talent. Do you believe years ago that all of the things you listed that they will be missing out on now didn't exist in comparison to the big schools? Sure those disparities existed. The difference was those talented guys simply weren't allowed to play at major schools so they found their way to the NFL by playing at the schools where they could play.
Make no mistake about if IF more talented players went to hbcu's you'd see more hbcu kids being drafted despite the "lack of development." And all of this talk of development completely ignores the fact that a good amount of the draft is based on projection anyway. How many times do we here talk like "hasn't scratched the surface" or " he's young for his age" or "he's only played football for x years" or "he's played x position but he projects as a (insert position) at the next level."
Truth is a good amount of these coaches that we think are better are just better because they have better players.
Yes, if more kids went to HBCUs, more of them would get drafted. That's just the law of large numbers. However, I guarantee you that the rate of 4 and 5 star kids getting drafted out of HBCUs and getting drafted in the early rounds would lag FAR behind the rate for kids that go to the bigger schools.