Is this true for everything? There are open-source games (a great example is Mindustry) that are available for free elsewhere while costing to buy via Steam.
If you're using a completely different distribution system/client like itch.io or the epic store, you're allowed to list them for cheaper.
This is the dominant reason that the Epic store isn’t taking off, because Steam is enforcing network effects. Devs can’t switch and so customers won’t switch.