Legally, it's still a license, it's just effectively impossible to revoke.

Edit to expand on this: A truly offline forever-purchase of physical goods can be re-sold. See https://en.wikipedia.org/wiki/First-sale_doctrine (this is the US-specific version, other jurisdictions may have similar doctrines).

American legal concept that limits the rights of an intellectual property owner to control resale of products embodying its intellectual property.

A digital "purchase" is usually non-transferable, even from GOG. It can't be removed from your own HDD once you download the installer, but there are still restrictions attached on what you can do with it, even if those are limited and hard to enforce.

If you back up the folder of a steam installed game that doesn't need steam to run, what's the difference?

Owning the copy in a legal sense doesn't affect most of the userbase tbh.