Because the main reason for companions' existence in this game is to f*ck em, and according to sources, dwarves, gnomes and halflings are the least "sexy".
You can 100% make your character a dwarf/gnome and still have sex with your companions. Can you please share the sources?
Yeah, but that's YOUR character, that's not the character that your character gets to sleep with.
I mean I am not as sure as the OP. They did include Lae'Zel as a companion who is, uh, not a looker. But it does feel like a shame that so many companions are either humans, or half-elves/elves (aka slightly prettier humans.) Besides that you've got one tiefling (human cosplaying as a demon) and Lae'Zel. Definitely feel like we could have used some of the shorties as companions.