When the new client was built, microservices were the hot new buzzword.

The new client is some weird plugins/services based architecture. Things that'd barely warrant their own class in a boring OOP-based UI framework are instead now "isolated" services. The reality of this isolation being that if one piece breaks, the whole UI becomes unusable anyways. Dozens of things that in another app would've been just a simple synchronous call now behave like remote procedure calls and messages, forcing all the complexity of distributed systems into a local application for no reason.

That's why it runs like ass, breaks if you look at it wrong, and your CPU draws more power when using the client than when playing the game at 200FPS.

CEF is + has been the de-facto standard when you have a native app and want to do a web UI.

It's not the only option, but it's the most mature with the largest amount of docs + stack overflow questions, so it's a "safe" choice.

If you peek into the native resources files of most games/desktop apps, you'll find a good portion of them bundle + use the CEF dll.

This is more a function of how mismanaged the project was at Riot (the iron client days, choosing Ember, etc.) which led to the current state of the launcher.