Because they have a very simple solution: offer their game through their storefront. Why is it Valve’s problem that the World of Goo developers want to forgo their popular storefront because they partnered with a company that forbids it as a requirement of funding?
The real answer of course is that nobody is obligated to help the other with their product. The issue comes down to consumers and what they want to support. I think Valve is being perfectly fair here and Epic is not. The Steam Deck is an open system, if Epic wanted to build a storefront for it, they could. They choose not to, because they don’t want to promote Steam Deck sales.
Isn’t it funny that the “run your own marketplace and keep all revenue” option that Epic took Apple to court over is already available on the Steam Deck from Day 1 and Epic chooses not to take advantage of it. It’s almost like company using a pile of cash to artificially tip the scales in their favour is perfectly fine as long as its them.
Everyone made their business choices here, and they have to live with the consequences.