can be a lot of things. could be the division/class they play in is not very good or well thought of, competition levels are considered less than higher classes/divisions, especially in bigger metro areas where you may have 6 or 7+ different HS class divisions. could be that they didn't/haven't gone to a bunch of prospect camps throughout their HS career, so scouts simply haven't seen them. could be that he's used all over the field because of the athleticism/talent level, but never stands out at any one position so doesn't get noticed with other prospects excelling at a single position...
anyway, you get the idea and i agree with your premise. we see and hear about the Top247, ESPN300, Rivals250....out of hundreds of thousands of HS football players every year, so there's always guys out there that are excellent football players that never get noticed until some program's scouting department finds them, offers them, and then you start to get the buzz. but it's so late sometimes, the kid never gets the opportunity to "improve" his rankings.
so while i will always say "give me all the 4 and 5*'s'....i don't scoff too much when a staff offers a kid that you've never heard of, never been ranked....especially when they offer that kid at a camp, or just coming off a camp, at that school. and it's not like some of those kids just show up at those camps, many times it's the program's scouting departments that invite them to the camps at the school.
anyway, long story short....agreed.