effort/infrastructure to host a download and display a webpage
Except that’s not all Valve does. Game files and updates need to be distributed, and that alone is a massive task at the scale Steam operates on, both the storage and transmission of data, and the operating cost of the CDN. Steam Cloud is also not free, it’s covered by the 30% so the players don’t have to pay for the service separately. Add to that the cost of sales where the discount is covered by Valve.
The EGS isn’t profitable either, it’s kept alive by Fortnite money.
Actually they can’t. Steam’s TOS has a “most favored nation” clause that forbids developers from charging less for their games on other platforms (at least this is how I understand it, I’m not a lawyer). From a small developer’s perspective, it sucks that they can’t unburden the player from the 30% where it doesn’t apply. From Valve’s perspective, that would turn Steam into an advertising platform for other stores.