I never thought of Valve as big enough to require contractors. I think this counts as a sign of their ongoing transformation to be more than a game/even software company as they now own hardware and a platform for it
Valve isn't some small indie company. They're not massive in terms of employee size, as they're only in the hundreds, but remember they have some of the biggest gaming tech stacks, such as Steam in itself, but also games like CSGO, which is arguably still one of the biggest fps games. No doubt they run those things solely by themselves without contracting some of the work outside of the company.
I think it makes sense for them to stay smallish and outsource projects to contractors. It's a nice change from SF companies who raise funds, then are beholden to investors who Demand that numbers go up - including number of people they hired. There's been a correction of that at least this year, with tens of thousands of people losing their jobs in highly funded IT companies.
They are now a corporation with hundreds of employees and equity in the billions of US dollars.
I don't think this is as simple as "require contractors". It's more along the lines of compensating specific people to work on specific things.