As another poster alluded to, digital goods aren't really considered property in the traditional sense. Digital property is protected under copyright (and other IP laws). The owner could sell the game, but then they wouldn't own it anymore (e.g. when one game studio buys another, they are buying the games as well). Instead, they grant a licence to use the game, which is how Steam works as well.

If Steam let you transfer your account to someone else (e.g. bequeath or sell it), then they would need this in the licence (which they could do in theory). Other than the logistics of that (especially how to handle people selling accounts - and the scammers that inevitably come with that), the AAA publishers are unlikely to agree to those terms. Ultimately the Steam licence is likely a compromise between Steam's vision and all the AAA publishers that wouldn't publish on Steam if they didn't get the licence they wanted. A bit like how Netflix doesn't really care if you use a VPN, they just have to enforce it so studios will let them use their content.