Randy (funny gamedev guy from YouTube) said in a recent video that he realized he'd spent the last ten years making engines to avoid the creative terror of action making a game. I'm paraphrasing slightly, but that's what it came down to.

"I thought if I made a really good engine, making a game would be the easy part!" I had similar thoughts when I was younger. Surely if I just upgrade my tools, the hard part will become the easy part!

Jonathan Blow says making engines is easy, because enginedev only takes a relatively small part of development — the game itself takes way more time and energy.

So his argument is, in the grand scheme of things, the engine is not that much work. (Since you're gonna spend ten years working on the game anyway, of course ;)

And, of course, the next level of procrastination is to develop your own programming language, which you will use to write the engine to use in creating the game :-) Definitely hats off to Jon for pulling it off - he had a lot of focus and some previous experience - and it also helps to have re$ource$ from past successful games. For many of us, the lure of developing better tools, rather than the end product, proves to be too strong to resist. At least Jon stopped short of developing own OS :-)

Jonathan Blow also makes games at a very slow pace; e.g. his Sokoban games. There are other factors at play, but his pace is consistent with the speed drawback of rolling your own engine.

And the thing usually is that what you want from your engine is the flexibility to be able to change things around easily so you can iterate and experiment on the game design itself. Sometimes a custom engine can give you that (especially if you're going off the beaten track) but often the tooling around the off-the-shelf engines is much better for it.

That's also why the Handmade Hero series took more than 600 episodes to eventually go nowhere.

That’s also why 99% of people building games with Unity or Unreal never get anywhere by themselves or just produce asset-flip slop, and then complain on Reddit about marketing being hard.

Programming an engine requires dedication, but pretty much every other area in gamedev require similar dedication to get to an acceptable result.