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Exactly, because an MRI is not a simple "shows problems" machine. It provides a very simplified model of certain aspects of the state of the body. We very often can't know if parts of that state are a health problem or not.

To my knowledge, studies have not shown any benefits of regular full body MRI's. You might find a problem, or you might find a non-problem and in the process of fixing it (aka operation / medication) you create a problem. Those two effects seem to balance out each other on average.