As a previous game dev. It was a combination of that. Also, you're often starting a project on tooling that is bleeding edge and that you have no experience with and isn't properly tested or documented.

Then game dev was always full of fresh junior devs with tons of energy, ideas and dreams, but who are coming from home brew where things like reliable, beautiful, readable code are unnecessary.

And tons of things get missed. I keep hoping that the one published game I have was accidentally built with debug symbols in it so it can be easily traced. Two of us on the project were heavily into performance optimization, and I absolutely remember us going through compiler options, but things were crazy. I remember one major milestone build for Eidos I was hallucinating badly when I compiled and burned the CD because I'd been working for three days straight with no sleep.