By charging 3% instead of 30%? Do you really think their servers cost $8.5b? Does the work to distribute a game and process payment equal 30% of the labor required to make a game?
A more advanced answer would be a cost plus profit model, so if it costs Valve $1 to transfer 1TB of data transfer (in terms of server costs), then charge $1.10 for 1TB. That’s obviously very difficult to calculate though I bet Valve has some internal metric of costs.
Valve today does the exact thing Unity was trying to do, charging a percent of revenue for providing infrastructure. Unity got raked over the coals for it.