Precisely. They never check that you are who you say that you are or that you are in fact still alive, so this "rule" is unenforceable. Case in point, many years ago I told Valve that my birthday was Jan 1st 1916, the earliest date it would let me select when I get prompts to input my age for mature-rated content. It still remembers that and autofills it for me on every age-restricted game page I land on in the discovery queue. If it were true, I'd be a 108 year old gamer right now, which isn't impossible but would probably raise some eyebrows at Valve if they ever had the intention of enforcing the "no passing down your account to other people" rule as it would be highly likely that I would be dead and my successors are the ones actually spending 7 hours on the weekends binging TW: WH3 and Stellaris.

It's the executors job to handle the inheritance, which is very different to transfer and sale. Insurances and services of all types handle inheritances, and they ask for documents specifically only available in such circumstances to verify it. It really is due to unwillingness on behalf of Steam