It’s not that simple. Proton implements the Windows API functions required to run a Windows game on Linux, but it’s not a CPU emulator. Emulating x86 on ARM at the speeds required by a game is virtually impossible.

If Steam comes to ARM / Android, it would have to be a whole separate ecosystem of games. But Valve is late to the game there since we already have several players on that market, not least the standard Google Play Store.