I believe they do because there are infinite possibilities out there. There's a good chance there's a parallel galaxy just like ours with small differences between them. And considering how little we know about everything that goes on outside of earth, we couldn't even say there aren't living beings on the planets we think we know a lot about. Lots of possibilities. It'd sound weird if you weren't to think about it from the infinite aspect of it, but there could be living beings made out of just rock for example. Our theories on what is required for life is limited to our earthly knowledge.