Steam has a very hard-core gamer feel to it. Just look at its interface. I do often admire the simplicity of Google Play Store. And I think a dedicated store for casual gamers is both a user experience and discovery issue. Steam solves neither of those because they need to sell "all" kinds of games and with that comes the baggage of years of market choices like DLCs, game "editions" and bundles. Google could just go straight to "Click, Pay and Play".