A big one is the lack of peer reviews and processes, including team meetings, that would slow them down. No PM, no UX, just yourself and the keyboard with some goals in mind. No OKRs or tickets to close.
It's a bit like any early industry, from cars to airplanes to trains. Earlier models were made by a select few people, and there was several versions until today where GM and Ford have thousands of people involved in designing a single car iteration.
IMHO the biggest thing is that they were their own customer. There was no requirements gathering, ui/ux consultation, third party bug reporting, just like you said. They were eating their own dogfood and loving it. No overhead meant they could focus entirely on the task at hand.