Valve make no profit on the consoles themselves right? And if the steam deck profit model of making their money off of games applies to games bought on any handheld, then why not let the other companies have the higher end market and all the research and development costs that making a new console needs. Especially because each new generation would need a refresh to stay on top.(more r and d costs) So why wouldnt valve just keep making their current steam deck that runs stuff well enough for them to keep selling games.
They made a whole new market segment, competing with others on the high end would be a yearly battle. Keeping things simple and making steam OS viable and trustworthy is a more long term approach while they keep the same machining processes longer term on the same hardware making the overall cost per unit lower for them as their not constantly changing it and investing R&D into new hardware.
They are competing on software as that is what they sale, games. Hardware they showed other companies it’s viable and succeeded there. If steam OS becomes open to everyone and works good, that would make a good platform for all these devices. And the steam store would be the obvious choice to buy your games.