a 4 or 5 star is such usually because they have multiple offers from top 10 schools (and likely more). It is not the result of an independent evaluation done by the recruiting services. They simply don't have enough staff to cover the entire country and see every player.
Thus, for those without high rankings, it is usually a function of not having big offers. In most of those cases the services (247, socut, rivals) never even see them play to "evaluate" them.
Having low stars for BYU recruits doesn't bother me in the least. Its what happens once they arrive on campus that is important for the types of athletes BYU gets.