They do...because that is easy. Remember that they are a business and the more info/stars/grades they can give out the better it is for business.
Watching game film and/or attending games is not easy. Having 100 RB's line up and timing them is super easy.
So they have a model they follow and it works for them. When recruiting sites first started, like Edgy Tim out in IL...that dude was actually hitting up 4-5 games every Friday to watch a quarter here and there and then write about the players and sort of rank them. It was a slow a$$ process that really only covered the Burbs of Chicago.
Rivals and 247 isn't going to invest that type of manpower and really, why would they? You might see a game where the kid sucks...or where he is amazing...you might see a game where the kid got in trouble that day at school and now he isn't playing that night and you have wasted your night.