Bats are mammals and they have a big overlap with humans regarding virus affinity. They get the viruses because they eat parasites and insects carrying these viruses. I saw a post yesterday where they were talking about some cave in Africa that people just cant go in because there are too many crazy viruses in bats in there.

Also as mentioned in other comments. Bats can just host a virus without getting sick from it. Except for rabies. Bats can still give you rabies, but they cant host that without being affected by it.