Why do crossovers do more harm than good???

Think I got down why unless it's an expensive parametric EQ that basic tone controls or none at all are better than graphic EQ but was thinking that if bi-amping latterally is supposed to be better than not bi-amping why don't more manufacturers make active crossovers and let's incorporate them into their pre-amps?As opposed to letting both amps handle full frequency spectrum?I have a set of bi-amp/biwire Gallos and was thinking of adding a sub.Why would a unit like a Bryston or other manufaturer improve the sonics more signal path and wirring nonwithstanding?If I did want to explore this option any other good manfacturers to consider?
Chazzbo