Why are managers allergic to finding a replacement for you when you tell them ahead of time that you won’t be able to work a day?

I hope this isnt the wrong place to post, but this has always annoyed me. If the managers are supposed to be in charge of scheduling, why do they make it up to the employee to find someone to take their shift or else they won't be allowed to miss? The employee doesn't know the availability of other employee schedules and is only able to ask the few employees they happen to be working with that day they ask. It makes no sense. Am I missing something?