Cause it's opensource, cause chromium engine is kinda standard for web, cause there is no better alternative to embed browser into your app.
Yes, Electron exists (like Discord), providing similar functionality, but it's more for running web-app on your PC. And it's for JavaScript developers.
But Steam - is C++ app, CEF - is C++ library, and with C++ you can do a lot more stuff, that couldn't be done with JS (or with much less performance)
Steam had VGUI and there was literally nothing wrong with it, and it worked for decades.
Issues that are entirely CEF's fault and have been ignored by Valve for years:
Well why not get these things fixed up-stream in CEF so everyone benefits from them being fixed? The whole dropdown menu thing is just a huge mess. Been causing me problems for actual years now.
Just asking based on how many issues CEF is responsible for.