Valve is not an ordinary company. They make a ton of money, have no outside investment, reinvest everything internally on R&D and keep very small. On top of that, they run completely flat management.

They're the idealized version of what a small company making a shitton of cash would be. They can afford plenty in terms of work-life balance.

Even at the start they were unusual as they were funded by Microsoft millionaires, and presumably had little pressure to release before "when it's done", and HL1 being a huge hit started the ball rolling allowing them to acquire the team fortress and counter-strike mod teams, picking up even more momentum.