It's more that no supplier can credibly claim that they can deliver it. If there's a rights dispute about a piece of underlying code for example, the terms can say what they like but they can't override it not being that entity's code to licence.

Valve certainly try and insert a presumption they can leave the title up, but it's not a given, and there's also not much to stop a developer from "updating" the title to a barely working basic binary.

That's also a risk with most other purchases, including purchases of physical goods. If the seller got the goods from an entity that did not have the right to transfer ownership or possession of that good, then the seller is not going to be able to transfer legal ownership or possession to you.

That's not what is going on in most of these cases though. In most of these cases the entity that provided the digital goods to the seller did have the rights to do so, but those rights weren't perpetual, so the seller could not sell the consumer perpetual rights.

Valve could protect against that by dealing only with developers who do have the ability to transfer perpetual licenses and requiring them to do so in their contracts. That wouldn't completely eliminate the risk because the developer might be mistaken or even lying but it could greatly reduce it if they are careful about choosing developers.