Because on shorthaul aircraft there are only two people qualified to aviate onboard, and if one needs to be in the bathroom - this necessitates putting an untrained person in the cockpit.
You need a great amount of paranoia to engage in such a practice literally thousands of times a day in the skies, all on the slight chance a pilot with thousands of hours of experience suddenly has a mental issue. Adding an untrained person to the cockpit introduces a whole new set of risks (cabin crew tend to have less pay, many have less experience than the people flying, they themselves may have issues, etc....).