To be fair, the difference here is that Unity is doing more than just an engine for a specific type of game. Rather, they are building lots of different tools for lots of different games.
For example, Larian (BG3) and Wube (Factorio) each have their own engine. They are specially built specifically for the games they are making.
Also, you say "have been kept modern" but even that is questionable. "Modern?" What does that really mean? It's "modern" enough for the game. Starfield was just released, and it was released without ray tracing.
And we are just talking about games. Unity and Unreal do more than just working on the engine for games. So yes, while it's a lot, it's not fair to compare the bespoke engine use by game companies and engines like Unity and Unreal.
How many such game development companies actually are there? I see a lot of aging engines out there, and companies that jumped to unreal, but I have by no means broad knowledge
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.
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.
