Tbf, when they coined those terms, they probably haven’t discovered like most of the variety of species yet, but that was the best term they had at the time.

Edit: Also: Venus Fly Trap does not have legs to move. It technically does move, but it’s still pretty much stationary relative to the ground. the 植 in 植物 (plants) basiclly includes the character 植, meaning “to plant”/“to establish”, so anything within the 植物 category cannot relocate itself (excluding via reproduction, spreading seeds, which doesn’t count for this purpose).

Also, doesn’t sea cucumbers move? I mean, snails are animals, theres no confusion about that lol.