Comment by keyringlight

My theory is that there's a period when a studio has huge early success (plus in the case of Valve, they started with huge amounts of money from being former MS employees) that lets them devote themselves to their mission of making games, before either mission creep or dilution with new hires occurs over time either from staff naturally changing over time or expanding. Another factor is that when aiming to 'go big' and realize what they can do with lots of resources, they need to partner/join with others that don't work the same way and will influence them.