Certainly not an expert but top studios like Bethesda and CDPR maintain their own engines with an order of magnitude less employees. They make games too.

CDPR seems to be switching to Unreal.

https://arstechnica.com/gaming/2022/03/the-witcher-will-retu...

They also produce games that are very similar to their other games, with similar mechanics, while Unity has to support a far more diverse set of games (basically anything 2d or 3d), and presumably has to continually offer a wider set of cutting-edge features.

Given the state of Creation Engine, I wouldn't say that Bethesda maintains their engine well. CDPR is switching to Unreal. CP2077 launch issues were mainly due to their engine not being capable of handling such large games.

Off the top of my head, there's Larian Studios (bg3), and Haemimont Games (ja3).

I know the devs of Hades have a custom engine and they are by no means a large studio

That's a 2D single-player game— it gets way harder if you're trying to emulate the feature set of Unreal/Unity

Source: game dev using unreal

id Tech is pretty solid

>aging engines

Gaming companies get a lot of crap for this - it seems unfair. The OS I'm using to write this is a relative newcomer at only 30 years old. Obviously, it has changed a lot in that time, but so have the engines. Most software doesn't get rewritten, it evolves.