At first glance your question, contrasting the likelihood of discovery for a stationary tree vs. a mobile animal, seems to present a valid conundrum.

However, let me explain why this “common sense” question is misleading: by definition an immobile tree remains in one place for its entire life, requiring any potential discoverer to happen upon the exact spot (among the entire area in question) in which it exists.

In contrast, an animal’s requisite movements to find food, water, shelter, mates, etc., ensure it must cover a much larger percentage of its habitat on a regular basis, thereby providing ongoing opportunities for the creature to cross paths with its potential discoverer.

To put it another way: sure, an animal can “hide” whenever it feels threatened. But, by hiding i.e. “staying still,” it is essentially using the same strategy the tree uses its entire life! It is the animal’s movement during the other 99% of its waking life that marks it as a better candidate for discovery.

Remember, while it may be harder to hit a moving target, said movement makes the target that much easier to spot in the first place!