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.
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.