Why do astronauts go into hospital when the get back?

Astronauts do not go into the hospital when they return to Earth.

They usually go into quarantine to reduce the risk of them introducing an infectious disease back to the population in the unlikely case that they were exposed to a microorganism on the ship or in space. NASA usually has them take a break of a few days to a few weeks to prevent the crew from bringing unknown disease back to Earth in the event there was an exposure during the flight. In the meantime, the crew completes tests and has physical examinations to evaluate their health and recovery from changes related to microgravity.