Legally, you own a copy of a book if you buy the book even if you don’t also buy the copyright for the book, so wouldn’t be allowed to make copies (beyond the single backup copy your allowed to make in case the first is damaged that you’re required to destroy if you ever stop owning the one you bought). This also applied to physical copies of video games, at least when they were new. In the US, this is part of what’s known as the First Sale Doctrine, but other territories typically have stone equivalent even if it doesn’t have a name.
About a century ago, book publishers tried putting a licence in copies of books and claiming that if you bought them, you were only buying the licence, but stopped after courts decided that you’d still bought a copy of the book and a copy of the text of the licence, but hadn’t become party to the licence. When game (and other software) publishers tried it later, they were basically just allowed to, even though there’s not really anything that makes the First Sale Doctrine any less applicable to games than books.
They don’t have a Linux version of their launcher, and don’t have a system to automatically configure Wine/Proton for games without native Linux support, which is most of their library. There’s nothing in the licence of Wine or Proton that would stop them just reusing Valve’s work, which is why third party launchers like Heroic can do it.