This topic really gets beat to death here. Those trying to give players and parents a reality check share that a certain high level of velo is necessary to get serious interest/$$$ with top programs. Others (often, those with kids who don't throw quite as hard) throw out examples of guys throwing below that threshold and "doing just fine" with Big School U.
It's pretty simple. Velo is very desirable (we have also spent countless discussions explaining why). College coaches look for guys with either current velo or projected velo. That gets you the most interest and the most $$. Once you get on the field, lots of things can happen. Velo is successful. Projected velo is reached and is successful. Velo is good but the other guy that didn't get the $$ is getting more outs. PC's find that you are more effective throwing 2-seam at a cruising speed instead of max effort 4-seam. Velo is good but too straight, can't seem to adjust and create movement. Velo arm gets fatigued but this is still the most effective P to roll out. On and on. So, many of these college guys throwing 84-87 were either guys with better velo or projected better velo when recruited.
If your velo or projected velo doesn't stand out among a very large pool of very good pitchers out there, you will not get an offer that stands out from a top school. Of course, there are some exceptions but it would be foolish to latch on to those and set "recruiting interest" expectations from them.
I think, the main reason that 90 is a magic number is that, categorically speaking, the pool in the upper 80's is just so much larger than the pool in the low 90's. Simple as that.