Bit of a straw man there, but I have to ask something.
Why do you consider aliens to be anywhere near humans in terms of playability?
I mean, is there any proper way to make a relatable character that is alien to the player and also the protagonist? Seems like a lot of risk that could push away the market if players can't find anything to relate to. Why not put the resources that could be spent on extra playable races into multiplayer?
Not really. Both are options for the player to create the kind of protagonist they want.
Because there are a lot of possibilities to see the Mass Effect universe through a different set of eyes by playing as one of the alien races. The Mass Effect races are unique, and it is a shame that we are limited to only humans to explore this universe with, rather than roleplaying the aliens a lot of people do relate to in this roleplaying game franchise.
Because SP and MP are done by separate teams by separate budgets, so one's resources doesn't impact the other.