Well, the game situation is that one of the PC's is an apprentice of the cranky ArchMage. Lots of stuff he isn't supposed to see, so the ArchMage locks it and delivers stern warnings. Creating and maintaining all those protection spells would seem like a lot of work for a wizard with a lot on his mind, plus the ArchMage doesn't really want to set off a fireball in his living room just cuz someone Knocked past the Arcane Lock on the study.
Sometimes all you really want, and need, is a good strong lock.
Sometimes all you really want, and need, is a good strong lock.