The mobile and PC gaming markets are very different, both in terms of monetisation and what games people expect to play.

If Valve wanted to get into the mobile games industry they’d basically be starting from scratch, and I don’t think it’s a market they’re particularly interested in.

You’re also assuming that buying a game on PC steam will also give you a license to play that game on android, which isn’t a given. I think many games have completely different monetisation models on mobile vs pc, so sharing between platforms like that wouldn’t make sense.