Epic Games CEO Tim Sweeney was asked by Verge why there is no support for the Steam Deck for Fortnite

Well if you're counting, I've got one.

No, definitely not but getting there! Last year despite slow production ramp up, about one million were sold. Source: a SteamOS developer on a KDE conference in autumn.

This year a financial analyst company predicted an additional two million until the end of this year but that was before the OLED announcement. Valve then recently said "millions". So I guess 3 million may be a somewhat conservative estimate now that the OLED model is out. 4 million if we're generous. It'll take a while until 10 million are reached, if they'll be reached at all. My memory is a bit foggy but I think Valve people said that the Steam Deck was intended to launch earlier but the Covid semiconductor crisis delayed the announcement. My guess is that at least a Steam Deck Lite will be announced first but the overall performance will stay about the same, so that could drive sales a bit before the eventual successor comes out.

It's not about Deck alone. Supporting Deck, means supporting all Linux machines and everyone else with controller.