For enlightened beings, Fun is when they see ignorance in action. So people thinking they are not suffering, that they are having fun can be very funny actually.
If you are asking why are monks so serious, is because they do see the suffering and delusion in many instances. They also know cause and effect which means they will absolutely take this opportunity of being a monk very seriously (not all of them). Their rules of conduct are quite extensive so keeping it takes effort. Not keeping the rules would be very dangerous as a monk, because it has serious consequences both in this life and future lives. If they are a monk, living on the Buddhas' inheritance, it has serious consequences if they are having fun in the lay sense.
Also worth mentioning, likes and dislikes are the mindstates that a monk wants to leave behind. No preference is the goal. Fun or freedom from suffering? The two are completely opposite directions.