Are you allowed to do a full body mri scan, yearly? would it be recommended if you can afford it? also, is the reason why people don't normally get full body mri scans is doctors might find things that don't necessarily need to be treated?

There will always be the day/week before any issue developed to a point where any test or imaging would be normal and give a false sense of security. The studied person would need daily testing/scans forever to avoid this. Also, many scans find variations of normal that may lead to other unnecessary tests. Many companies offer broad comprehensive exams, but the value is often questionable.