It is orders of magnitude easier to write an game engine for yourself than it is to create a monster like unity or unreal that needs to appeal to everyone and support every kind of game.

If we are talking 2d, it can be months to hack together a basic engine. 3d can be a bit harder but far from decades.

Thing is, if you designed your engine well and implemented great tooling, it should make it faster to implement the actual content of the game.

So upfront cost to be faster later. At least in theory. Obviously you might end up with subpar tooling that is worse than what a commercial could offers. But if you do something like an RPG with a lot of contend, every bit of extra efficiency in creating that content can help a lot.

Now, obviously from a purely commercial standpoint, not using an established engine makes nearly never sense. Super risky. Hard to hire outside talent. You are only justified when you have very, very specific needs that are hard to implement in a generic engine.

Also for us with an ADHD brain, hard things tend to be easier and easy things very hard, so yes the extra mental stimulation of writing an engine can help.