I think one problem is that although ARX packs are pictured on the game’s Steam page, “ARX” doesn’t appear in the text, so you can’t control+f for it.

But they already can and already do. For example If I wanted to buy ARX for Elite Dangerous, you have to go through Frontier’s website to purchase it.

You can buy ARX on Steam now, but you don’t have to.

https://store.steampowered.com/itemstore/359320/browse/