Star Control 2 had a veritable collection of completely unhuman species like the Ur-Quan (who look somewhat like sentient Thresher Maws), Chenjesu (crystalline entities), Umgah (heavily genetically-modified blobs), Supox (sentient plants), Mycon (sentient fungus), Ilwrath (arachnids), Slylandro (gaseous entities from a gas giant), Zoq-Fot-Pik (a symbiosis of three bizarre species), Orz (weird fish-like things), Dnyarri (looking like brains on frog legs). But you never got to encounter the specimens of those races in person, only on static viewscreen. Implementing a similar array of races in a Mass Effect-like game would be a lot of pain for the developers and animators.
If Frostbite engine allows to animate such species with relative ease - why not?