No reason Valve would sell anyways. They don't reveal much number wise, but there's no doubt they're one of the biggest companies in gaming. They probably have a larger revenue than the Xbox divsion of Microsoft honestly, Steam is a larger platform and the market place is a goldmine for them.
Microsoft Gaming revenue 2020: 11bn USD.
Valve (estimated) 2017: 4.3bn USD.
That is as close as we can get.
Imagine not having GamePass on your console...