Plenty of "serious" games are made in the engine -- Genshin Impact, Hearthstone, Marvel Snap, Among Us, Pokemon GO, Beat Saber... is there a reason those games are worse than something made internally?
Yes, the point is that having an internally developed game would mean that when issues and inconveniences are identified, they force valuable improvements/fixes to the engine (which benefit the product and others) instead of workarounds by the external game developer in their particular game.
it's possible to make good and big games in Unity, but you can't rely on many Unity features in that case. Like, I'm sure there is barely any "Unity" in Genshin Impact, I bet they replaced most of the engine with custom tooling.
It's been a while, but I worked on some Unity stuff in the past (I had a decade of C# experience back in 2013), and this is what I saw for a lot of mid-size-to-large games (but nothing AAA, and definitely nothing compared to Genshin Impact!).
On a lot of those, the engine is more of a bare-bones renderer: only one Scene is really used, all entity instantiation and level loading is done by a user-land script. Some physics/collision was very custom too. Editors were also almost always custom, on larger games you write custom tooling, mid-sized ones preferred packages from the asset store.
Perhaps things are different now. Character movement was 100% offloaded to packages, but now there are more capable systems (I haven't kept up). But I still see a lot of chatter about how to do this kind of thing in forums, etc. Anyway, that's what I remember seeing, so: IME and YMMV, of course.
All those don't use in house tooling for lots of stuff. This is a situation similar to android were some libraries from Google are useless basic implementations. Pointing out that android is widespread does not invalidate the point.
Also pouring enough time and money and resources in product will solve issues eventually. But does not mean things could not have been better or cheaper.