Because retail likes to infantilize their employees, and in their eyes, if they make it harder for people to take time off then less time off will be taken. Theoretically.
See also: How the time off request system is easy to ignore allowing leaders to just ignore the time off requests until they become auto-denied, requiring TMs to seek out their leaders (who they may not overlap with during working hours) to notify the leaders of the time off requests, which still allows the leaders to ignore said requests.
This may or may not have anything to do with why I'm an ODTM at the moment and pointedly ignoring the fact that my ETL keeps FUCKING SCHEDULING ME even though I'm not supposed to be put on the schedule, just like he did when I would request days off last spring.