No clear reason? It’s because the thing can successfully make more of itself before dying. That’s literally it.

See also: ticks

No animal has reason to exist.

As an animal myself, I’d like to object.

Okay, we’re listening. First establish some framework of “reasons to exist” so we understand what that even means, and then tell us yours. Centuries of philosophy haven’t gotten past the first step and you probably won’t either, but I won’t stop you from trying. Okay, go.