Because hospital staff have better things to do than baby sit every person that walks in? They are pretty well known for always being overworked already.

Then the door will always be locked, unless the MRI is being serviced, as the magnet is always active. Kinda kills the point of the machine, no? That said they could put in more safeguards for sure. Though you would think all the signs on and near the door, and the extensive explanation you get, would be enough. But here we are.