One key difference its revenue: Allen often mentions how any piece of code brought money to the company, simple apps that are 100% free today would have been sold for tens of millions back then. Same with hardware, which is why Apple could get roughly 1 million (in today's dollars) to leave the garage.
Basically both companies were in the right place at the right time: right before the PC revolution began. A YC-equivalent at that time would've appeared around 1982, after IBM launched the "official" PC with DOS and the ball was already rolling.
That was the time when a lot of other startups appeared, like Amiga, Adobe, Lotus, Silicon Graphics, etc...
Note that none of those companies were as successful as Apple and Microsoft were, in fact some went chapter 11 and disappeared.