Valve can't sell this at a loss because they don't have anything else in their portfolio to offload these initial losses while waiting to gain market share. They literally only have Steam, which in of itself is just a service.
They are not a multimedia giant like Sony.
They are not a trillion-dollar value conglomerate like Microsoft.
They are not legacy IP merchandise holder like Nintendo.
Valve is just Steam, and a couple of games they published years ago.
>but most of Sony/Xbox games are on Steam
Which they pay these publishers their share. Their leverage is the fact the majority of players prefer their service, but not on profit. Sony Playstation still profits on their own store in their own console that still sells in the multi-millions.