That's purely profit driven. The biggest difference to other corporations is that they're privately owned, meaning they can pursue long-term strategies instead of short-term ones. Publicly traded companies have to pursue short term strategies because otherwise investors get itchy and want to sell. Doing something that costs money and will yield results in ten years is a big no-no for publicly traded companies.

Everyone and their mother who makes any computer has to pay money to Microsoft because they put Windows on it. There are only a few outliers, most notably Apple and a few vendors who put Linux or no OS in there.

Valve doesn't pay them shit, meaning they can sell the device cheaper, thus getting more customers. That's the immediate gain. When you provide a gaming OS that you want to offer to others, you're also the one getting paid for providing support. That's the long term profit.

And through it all, as a nice bonus, they stopped being reliant on a single vendor and gained unbelievably great PR from a group of gamers.