There is a whole team clustered around an MRI machine and tying up the machine is effectively tying up the team.
If everybody got an MRI every year on the off chance they might have something then the machines would not be available to those that actually need them (because they have symptoms that require an MRI to narrow in on a cause).
And that's besides the whole fall-out further down the chain in case something does show up that does not generate symptoms and even further down the chain where probably un-necessary operations will be performed on symptom-less patients.
Doing the right amount of screening is a really hard problem. Getting an MRI done every year just because you can afford it is probably not even near the right amount.