They need to do that because, in some sense, they're competing with Gaming PCs, not really with Gaming consoles. Gaming consoles sell their consoles at a discounted price because they can recoup a lot of it when selling games. Steam can't have a markup on games because they share their marketplace with other PCs.
Steam has a very high markup compared to its competitors like Epic Games Store.
I would assume it also has to do with if not fundamentally manifesting from Steam being an organisation of technologists. They don't want to put out a project which has a worse operating system than their workstations.
You could still offer this, similar to the ad tier and ad free tier of a kindle, or a carrier locked phone.
$799 for a locked down version, $1049 for an unlocked version. Opportunity to pay $300 to unlock it later at any time. 5% discount on purchases on a locked device.