This is covered in the video I linked…

“Valve is not a publicly traded company. Being a publicly traded company adds a bunch of headaches and it [doesn’t] really solve any problems for us… It [would mean] that control around decision making now involved third parties… The whole point of being a privately held company is to eliminate another source of noise in the signal between the consumers and producers of a good.”

Valve are absurdly profitable. What purpose would being public serve? They don’t need to raise extra capital, they don’t have shareholders to provide liquidity to, they don’t need the brand recognition, they don’t need stock for acquisition currency, they don’t need it to attract or retain talent, Gabe doesn’t need an exit strategy when he’s already cash flush to be buying mega yachts.

For every reason companies go public, none of them would benefit Valve.