FTA:

The request comes after the manufacturer discovered two aircraft with missing bolts in the rudder control system, raising concerns about faults across all aircraft.

Inspection intervals are based on expectation of damage over time, not to verify if the installation procedure was properly followed.

Design requirements for airplane parts that experience rotation or are part of control systems are regulated to have locking features to prevent loose bolts from happening. If the initial installation was done improperly it could be a failure in quality control at Boeing. Or if they were installed properly but weren't designed with sufficient locking mechanisms it may be an improper design. Either way this could turn into an Airworthiness Directive which is when the FAA steps in to ensure safety.

Yes and no. Airplanes are complex, the 737 max especially so. To check every bolt after every flight would be impossible, that would require every plane to be taken to a certified shop taken apart and inspected and put back together after every flight. There are inspection intervals and lots of scheduled maintenance that regular airline passengers aren't aware of. We only see the planes we fly on not all the ones temporary out of service for maintenance. These bolts would likely be checked during some scheduled maintenance but it seems that the bolts may be failing prior to the scheduled check, or they were never installed correctly. It's not likely a concern to passengers, the FAA (and most other global air regulators) requires all airlines to follow all manufacturer guidelines and recommendations within a certain amount of flights after an announcement like this. Also modern airplanes are very safe one missing bolt probably won't cause and immediate problem during flight.