People really don't know how engines work and it really shows every time this conversation comes up. Unreal 4 is *the* engine on the market right now for a ton of very good reasons. MS has absolutely no incentive to try and build out a multi studio proprietary engine when Unreal exists - its pretty much all downside. You get versatility, power, ease of use, and maybe most importantly, all the open source improvements made to the engine by the wider community, so all three of those keep getting better.