Valve addresses this very question in the Handbook for New Employees:
Q: If all this stuff has worked well for us, why doesn’t every company
work this way?
A: Well, it’s really hard. Mainly because, from day one, it requires a
commitment to hiring in a way that’s very different from the way most
companies hire. It also requires the discipline to make the design of
the company more important than any one short-term business goal.
And it requires a great deal of freedom from outside pressure—being
self-funded was key. And having a founder who was confident enough
to build this kind of place is rare, indeed.
Another reason that it’s hard to run a company this way is that it
requires vigilance. It’s a one-way trip if the core values change, and
maintaining them requires the full commitment of everyone—
especially those who’ve been here the longest. For “senior” people
at most companies, accumulating more power and/or money over
time happens by adopting a more hierarchical culture.