I can, because they really don't seem to. They aren't a publicly traded company beholden to corporate share holders who only value profit. They have a history of behavior which suggests they care about PC gaming and not a heck of a lot else. Arguably they don't even care about the money that much. Did you know you can sell Steam keys on other store fronts and Valve won't even take a cut?

Want and ability are two different things.

Valve released a phenomonal but imperfect update to the Nintendo Switch which has sold about 1% as well. If Sony released a PlayStation Phone tomorrow, and failed to update it for years despite massive interest, then you'd see a SteamPhone.

Not to mention the fact that they might be able to tap into a market that practically doesn't exist anymore: paid mobile games. Like, actual, "I pay you $5-40, you give me a complete game". People are in the habit of buying games on Steam. Half of the top-20 most played games on Steam right now cost money. SteamDeckPhone's sales would probably be disproportionately people who already use Steam on the PC and/or the Steam Deck, so it would be a significantly different audience than the broader mobile gaming market.