>last major argument that I am familiar with for simply using what already exists

I was interested in gamedev as a kid. I always went with a custom engine. You're right, I never released/finished a large game.

But now I think I was just into game engine dev. Every time I finished my game engine and started working on game content I ingredient got bored and went to another project.

So I guess this is the reason, engine development is fun :).