There are so many high school players out there that Rivals, Scout, etc. can't see and rate them all.
When a player has no stars it just means he wasn't looked at and rated--it doesn't mean he is (or isn't) a decent prospect. So, when a P5 school offers an unrated player over the summer (because they saw him in camp, noticed him when scouting or looking at another player's film, etc. and liked what they saw), the star-assigners then decide to catch up and look at the player's measurables (ht, wt, speed, etc.), number and quality of other offers, and junior year game film and finally assign a rating. That usually means the player will at least be a 2-star (default minimum for a P5 commit) and sometimes a 3 star. Presumably all 4-5 star guys have already been rated since they were big HS stars as juniors. It is just a fact of life that lots of 2-3 star-quality guys don't get rated by the end of their HS junior year because the star-assigners haven't seen their measurables and film. There are just too many kids in that category for that to be possible.
Most ultimate true 2-star P5 pickups around LOI day are a few guys grabbed late to fill a recruiting class as plan B's or C's. Two stars taken before their senior season often wind up as 3-stars by LOI day based on a senior season that reflects what the coaches saw when they camped the prior summer.