Originally, ME:A was going to be procedurally generated, which would of allowed for a much greater variety of creatures (and flora/Vistas) but got changed to hand rendered assets. Re-using assets then becomes cost effective.
I suppose they could put out a DLC expansion pack for flora and fauna if it looked like there'd be a demand for it. I'm glad they put more time into hand rendering and storyline than making an exciting variety of flora and fauna. No Man's Sky was a huge disappointment past just the procedural generation possibilities.
Shorter answer: Because all of them try to kill us. 🙂
