That kind of monism entails that virtually all thoughts are entirely misleading, which would seem pretty counterproductive. Yes, physically, we're material things. But other patterns emerge from the physical, and some patterns are rarer than others. Life is rare in the known universe, so we mark the distinction with concepts.

Likewise, animals emerged from simpler forms of life, and people evolved from animals, so we mark those transitions, too, with concepts. Animals feel pleasure and pain, for example, whereas most scientists think plants don't.

If plants are more animal-like than we think, we're in big trouble because we'd have nothing left to eat while maintaining our moral standards.