The ranger is different, because a) it's meant to be a combat mechanic and b) it's so stingy with the actions needed that it breaks immersion for many players. ("Wait, so if I had a trained dog, it could just do a full attack, but since it's my companion, I have to use my actions?") The latter is pretty clearly an experiment with holding over some 4E ideas, and... yeah, bad.

But familiars have so many other uses! The imp alone can turn invisible; turn into a rat, raven or spider; see in magical darkness to 120' (twice as far as an elf or dwarf's darkvision), and has magic resistance. Or sprite familiars have heartsight -- they can tell someone's alignment and emotional state with a touch. All of that is awesome, and at least some of it could be adopted to BG3.