Generally, I've seen the exact opposite. People who code their own engines tend to get sucked into the engine and forget that they're supposed to be shipping a game. (I say this as someone who has coded their own engine, multiple times, and ended up not shipping a game--though I had a lot of fun working on the engine.)

The problem is that the fun, cool parts about building your own game engine are vastly outnumbered by the boring parts: supporting level and save data loading/storage, content pipelines, supporting multiple input devices and things like someone plugging in an XBox controller while the game is running and switching all the input symbols to the new input device in real time, supporting various display resolutions and supporting people plugging in new displays while the game is running, and writing something that works on PC/mobile/Switch(2)/XBox/Playstation... all solved problems, none of which are particularly intellectually stimulating to solve correctly.

If someone's finances depend on shipping a game that makes money, there's really no question that you should use Unity or Unreal. Maybe Godot but even that's a stretch. There's a small handful of indie custom game engine success stories, including some of my favorites like The Witness and Axiom Verge, but those are exceptions rather than the rule. And Axiom Verge notably had to be deeply reworked to get a Switch release, because it's built on MonoGame.