I know not every game has a mobile port, and this the way it works for some other media too buying a physical copy of a book doesn’t entitle you to an ebook version and vice versa. On the other hand, a steam game can be played on Windows, MacOS, or Linux without restriction provided it’s ported or works with Proton.
In any event it still rubs me the wrong way to know I bought Slay the Spire on Steam, and Steam has a mobile presence, and StS has a mobile port, but that still doesn’t end with me playing StS on mobile without buying it again.
Hopefully the recent court stuff with Epic and Apple will mean Valve could start putting up their own mobile launcher on iOS, as I imagine they wouldn’t see just Android as worth the effort.
I think the likelihood of this happening is continually increasing as ARM-based and similar RISC processors become more and more powerful and x86 emulation continues to improve. There’s a very real chance we’ll see ARM-based steam deck competitors in the next couple of years, and at that point it’s basically just an inevitability.