MRI is a finite resource. They are big, heavy, expensive to run and maintain, and require large amounts of helium to cool. The scans on them take a long time, so throughput is low. Monitoring is hard, because it all has to be done with stuff that doesn’t stick to, or heat up from exposure to, a huge magnet. MRI technicians are in high demand. Many hospitals don’t have MRI overnight.

MRI is never routine in healthy people.