The game store doesn't need a lot of employees. A few years ago it was reported that Valve only needed about 70 employees to run Steam while it generated billions of dollars in Steam fees (30% per game). It's basically free money for Valve. I bet the situation is similar for the AppStore and Google Play.
Though Unreal Engine does indeed need quite a few developers. Additionally, using UE is much cheaper (5% on games exceeding 1 milion USD gross revenue) than using Steam (30% on every game). So they not only need more developers than Valve, they also earn less money.