I get where the OP is coming from, because MS has the resources and the history (and now several renown "engines" either developed in-house or in their roster through acquisition,) and when you look at the games made with the tech MS owns, you think, "more of this, please!"
But the term "engine" is way over-esteemed in the importance of developing a game. Making games look pretty and run smoothly, that's almost the easy part of designing these days.
What's much more important is the development suite surrounding the game engine, and the assets services and plug-in/middleware provider accessories and especially the technical support that is available for users of the engine. That's where Unreal Engine and Unity earn their adoption. A developer can create their own engine, but then everybody who works with that engine needs to come to that developer to understand how to work with it or to make new parts of the engine needed for the project. Game development suites and middleware services are about making game creation feasible through solid design and performative tools; how badass the final game can be, that's generally up to you.
Unreal Engine 5, for example, is a little bit of Nanite+Lumen graphic wow-factor on the surface and a whole lot of ease of direct ZBrush sculpt/photogrammetry scan import, and a believable and tested GI solution to ideally eliminate hard-bake lights/reflections, and collaborative design improvements such as One File Per Actor or Data Layers, and a sound system with control over DSP graph generation and an IK Rig that can allow additive adjustments to pre-existing animation. and streamlined workflow with an easily browsible Content Browser, and stability, quality, & performance improvements, and on and on...
Doom Eternal, Forza Horizon, CoD, these are all impressive-looking games in part because they have elite-quality engines that run high-quality assets at near-flawless performance with robust features in online and other functionalities... but all of that was just the foundation to integrate the work of hundreds of people, only a few of whom were creating the "engine". If you just handed these engines out to other developers as is, you would not get the next Doom Eternal, Forza Horizon, or CoD just by way of the engine being awesome.