It all comes down to money.
If Epic was running their own platform, and 3rd parties didn’t want to abide by their rules, then Epic would give those 3rd parties the finger and tell them to F-off the same as Apple.
Each is trying to grab as much profit as they can while running into the issue of other hands grabbing from the same basket.
The classic case of one’s greed versus another’s desire to be greedy.