I thought radiologists need to know what to look for in order to diagnose something? Do they brute force every potential condition in the body that can be detected with an MRI?
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.
No, when they read a scan, they're supposed to read everything visible for every problem. Think of it this way: if you break your leg and they take an MRI, do you want the radiologist to miss a tumor because he was focused on the break?
I suppose a full body MRI would be very expensive and take a lot of time to read.