Even for the Steam Deck it's clearly a big project to get a critical mass of games working on a non-PC device, and that's much easier than Android (only one model of device, no need to port code to another OS and platform, no need to port game interface to touch UI, etc). Ask any Android game developer and the support & cross testing of the zoo of device models and OS versions is a huge part of the work.

Plus on Android they'd be trying to convince Steam publishers to enter a platform where users don't pay for games but rather watch ads or make IAPs, so they wouldn't really get the right kind of paying customers from there.