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.
Usually you would be playing a windows x86 build via Proton on an x86 (or emulated x86) system, but AFAIK Proton doesn't have a backend for iOS/Android (which are ARM), and steam won't even run on an ARM linux ubuntu. Devs most likely would refuse this because converting a game to Android/iOS is a whole new build, they might not have been using an engine like Unity/Godot that simplifies adding Android/iOS as build targets, they might not be willing to redesign UI elements for a platform that's touchscreen-first, they might not want to hire additional QA/play testers for mobile devices, and if they are willing to do all of that, they might not be willing to give it for free for users who bought the x86 windows build, they might even want to make the game's revenue model ad-driven or pay-to-win.
But you can ignore all that and buy a steam-deck like device, or download the "Steam Link" app and stream a game from Steam to your mobile device.
I'm not asking for everything to be ported to mobile, just that if there is a mobile version already made that I can play it through steam since I already own that game