alt Hacker NewsThe big difference is that the big game engines have to cover all sorts of genres and scenarios, which often results in bloated "jack of all trades master of none" code compared to engine-layer code that's highly specialized for exactly one, or few very similar games.
If building a custom commercial game engine these days... A team is 100% focused on the wrong problem, as the game-play content is what sells. Customers only care about game-engines when broken or cheating.
Godot, Unreal, CryEngine, and even Unity... all solve edge-cases most don't even know they will encounter. Trying something custom usually means teams simply run out of resources before a game ships, and is unlikely stable on most platforms/ports. =3