###Security
Due to security reason all personnel have to go through thorough screening and checks
These checks do not end all of the possible security issues. They just ensure that a single individual or ar small group does not pose a significant threat.
But what if a group of individuals become the threat? Physical force is still a thing, and if a number of individuals mutiny or revolt, they could be dangerous, either by attacking the loyal crew or accessing critical parts of the ship.
Not to mention that people can become very imaginative when it comes to improvised weapon design.
To put an example, we already have places were people are very careful screened to avoid them inserting weapons in a controlled environments. Yet improvised weapons are made (shivs and the like), revolts do occasionally happen, and the guards do have weapons and/or have support from armed security forces readily available.
Imagine the situation in a spaceship where external support is not available and everyone on board knows about it.