Comment by flohofwoe

Because 90% of the work on an engine goes into the tooling (DCC tool exporters, asset pipelines, editor tools, ...), e.g. the "actual" engine runtime is just a small (and frangkly: quite trivial) thing dangling off the end of the asset pipeline. And game engine tools development is a never ending rabbit hole that's never quite finished (UI tools development got much easier though thanks to Dear ImGui).

IME two years sounds about right starting from scratch until getting everything into a state that can be called a basic game engine (if a bit optimistic). You can start earlier building a game on the in-progress-engine of course and tbh that's the only way to not develop an entirely useless engine. But that way you'll also end up with an engine that's mostly only useful for that one game, and progress on the game will be atricious in the first year or so. And of course developing both side by side means spreading the butter even thinner.

In the end though I have seen at least as many game projects fail using an off-the-shelf engine (in that case: Unity) compared to using an inhouse engine. The decision to use inhouse tech versus off-the-shelf engine doesn't make or break a game, in the end the failure always lurk within the team (but the engine is usually blamed first when things went south heh).