You could ask how humans evolved in a world that is mostly water and humans drown so easily. They are also present in regions surrounded by water? How did they get there?

They developed in places that are not water and moved to other places through water by occupying small spots of non-water while in transit.

Same with the virus. It developed in places that UV doesn't reach, like insides of larger living things, and travel to other habitable places by exposing themselves to UV only very briefly while being somewhat shielded by water in breathed out droplets.