Valve doesn’t set the prices for any of the products you buy through their store. The game developers and publishers do.
The exception is valve developed games which are mostly free to play and make money on useless cosmetics. Most of their successful games are built on mods that are only possible because valve takes the very consumer friendly position of supporting and encouraging modding of their games.
Hell, they even allow and promote fan made remakes like Black Mesa and unofficial sequels.
If valve is a monopoly, it’s only because they’re the only corporation in the pc gaming space (OK maybe include gog too) that respects their customers. They’re not perfect but they’re orders of magnitude better than the competition.