My theory...CMs are statistically insignificant to overall attendance, but are statistically significantly for areas that have "capacity deficiencies." Take Frozen and the 900 per hour. 10,800 seats in a 12 hour day. 3500 maingaters (I just picked a number, no idea how many actually exist) would represent about 11% of Epcot's daily average attendance, but 32.5% of the daily capacity for the ride.
In the case of the MK, I suspect that FP+ has upset the apple cart. Day trippers are choosing MK more frequently, and less park hopping (due to late FP availability in the lesser parks) so the "in park, at this exact moment" attendance is climbing , and with thousands more people ejected from queues (when HM, Pirates, and the other rides were added to FP), the park just wasn't designed to have that many people in walkways, etc. They were meant to be in queues. CMs are the only ones that they have the power to restrict access. If Epcot were to get 3500 maingated, maybe MK gets 5000, and that can have an impact for curb space for parades, tables in a restaurant, the line at Mine Train or Peter Pan, etc.
I've heard that cast members only make up somewhere between 2–5% of the daily audience. Just trying to verify that…
Assuming that's correct… Magic Kingdom, worst case, 2648 cast members come to play, on average. I can't imagine that everyone of those would be getting people in.
So maybe 1000 people a day? Even 2000?
Either way it's statistically insignificant. I could understand if it was 25% of the audience or 50% of the audience… But five percent? 2%? It's ridiculous.