Some storefronts are just there to sell you the game without DRM too. We have GOG and itchio in example. Or they sell the game on their website just letting your download the game, which would be like you got it on the USB stick or CD. And even for shops like Steam, they can sell the game on other platforms and websites a Steam game key to unlock it in Steam, without them paying ANY money to Steam! And all of that on top of what you mentioned with physical media being an option.

The “only problem” is, that most publishers and developers don’t do that.