I don't know how many kids are tracked by recruiting analysts, but it must be well over 1,000. It's impossible to see all these kids, much less compare them. And, even if they could watch everyone and accurately rate them, they can't predict how they will perform in college. So, they gather up all the information they can, including team interest, and give them a rating.
Just look at the NFL draft. Teams spend countless hours evaluating pro potential for kids that are 21 or 22. That has to be a heck of a lot easier than evaluating 16 & 17 year olds. For starters, they are only looking at a few hundred guys, instead of thousands. Yet, the NFL misses on draft day all the time!
Recruiting ratings are not close to perfect, and also not close to worthless.