Nobody seems to consider that doing it yourself, requires you implement it at least as efficiently as the commercial engine did, otherwise you're just creating a worse-performing implementation that seems to behave just like a bloated engine does.
The 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.
The 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.