There are a whole bunch of kids who develop "late" and are therefore not recruited by the D1 schools that would otherwise be on there radar. MANY of these kids become every bit as talented as so many on D1 rosters even though they end up at other levels. I'm not talking about the 8-10% of D1 guys who go on to get drafted but the rest of them, particularly those at schools other than the power 5's.
There are a whole bunch of kids who don't recognize baseball as being their target path definitively enough until later in the process and therefore have a weak recruiting plan as it relates to the typical D1 timeline. There are many who have a weak (or non-existent) recruiting plan in general. There are many who are not properly guided, cannot afford or do not, for a variety of other reasons take the right steps to be seen in time to be recruited by D1 schools.
There are many who are not willing to explore college options in large swaths of geographic regions. Therefore, they eliminate themselves from possible D1 options. Many of these players are more than capable of playing at some D1 programs.
There are many who end up at D2, D3, NAIA due to grades, location, and a host of other reasons who end up developing into D1 level players but choose not to change schools mid-stream just for baseball when they have made significant inroads with their academic pursuits at their current non-D1 schools.
There are many JC players in hotbeds who assume that, because they are getting so little D1 attention in their own hotbed region that there aren't D1 options for them elsewhere. So they go another level.
There are players who play the wrong position at the wrong time as it relates to matching up with the needs of the few D1 schools they may be targeting. So, they go another level.
There are players who's academic focus doesn't match up with what is offered at those D1 schools who express interest. So, they go another level.
I could go on. There is plenty of overlap. "Unicorn" isn't accurate at all, in my observations. It is certainly true that the Power 5 schools get more top shelf draft-worthy players and, yes, there is certainly general hierarchy. But aside from that, there is a fair number of players who's skill sets overlap across all levels.