I think it is also because of the amount of knowledge that has to be imparted. Years ago it was relatively simple to teach new CMs both Disney history as well as park operations.
Now, there are a LOT more Disney media items out there - more characters, more shows, more games, more cannon, etc. and none of the old stuff is going away either (despite the efforts of Disney on certain items). To add to that, you have certain guests that are into certain niche topics that a CM is not likely to know about. I'm sure there are some guests that come up to a CM and start to talk about VMK, maybe even mention something like "falling chairs" and when the CM draws a blank, they are incredulous that they don't know what it is about.
Also, the parks themselves are a lot more complicated. Instead of just training a CM on park hours, park passes, and paper fast passes, now they have to know park reservations, the app, Lightning Lanes, virtual queues, dining reservations, and a host of other topics.
It is a LOT of training for somebody that is new to the whole Disney / WDW experience.