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 wish phones and handheld games were completely separate again tbh. even these annoyingly large-screened phones they made annoyingly large-screened for Gamers
are imo too small to really get into, just let phones be compact dedicated communication devices and if you really want to game on the go get a tablet or steam deck or whatever 
but anyway can’t you just pirate the mobile StS?