How do you know this? Everyone kept saying he could wipe out half of life in the Universe with a snap of his fingers. No one ever said only sentient life, just life. To me, it seems like an assumption to assume that means only sentient life.
Did they show us any plants or animals being eliminated? In fact they showed us lush grasslands afterwards. Thanos motivation was to stop people from starving and save the planets from being consumed. That dude is the Ultimate Captain Planet.