Because technically, they didn't. As they say in the documentary, the version of the game that they made after the first year or so was bad, and they acknowledged it, and they changed their processes and introduced the "cabal" - so they basically paid that first year to gather experience. And what's important, their publisher Sierra was okay with it.
I don't see that happening today, to be honest, in "AAA" titles at least. Budgets have gone through the roof, and the publisher won't give you a plan B, they will force the developer to duct-tape what they have and ship it no matter what. And the game can always be patched later, right?
One thing I have found extremely interesting was how Valve hit the jackpot with many early hires.
It was literally people that often didn't even have a CS or let alone gaming background and yet so many of them proved to be relentlessly resourceful, creative and hard working.
The guy who wrote most of the HL code wasn't even a developer and at the time was studying to become a lawyer or accountant iirc.
I can't but think that timing, founders but also straight up luck played an absolutely crucial factor.