Quoting: pbSo what's stopping them from disclosure?

Nothing. And plenty do.

Quoting: pbAre thy ashamed of using "a tool"?

Shame has nothing to do with that.
But you can see from the absolutely braindead "outrage" over Swen Vincke's very reasonable stance on AI usage why a developer wouldn't even want to talk about it at all.

Anyway, this is about Steam enforcing disclosure when it comes to generating content that is "consumed" with the product.
And as said already, that only makes sense with parts where that can even be reasonably checked.
You can't check if someone uses Mistral in their IDE to help debugging, as a more efficient "Google" with more context, not at all, or writing half their code with it - the dev can tell you what exactly they do, or not.

Quoting: pbIt's also our right to know if the code was produced using genAI,

There is no such right.
You are hallucinating harder than ChatGPT on its worst day.

Quoting: pbWe have obligatory lists of ingredients on food products, cosmetics, detergents - not only because of allergens, but simply because we (the consumers) have the right to know what kinds of shit went in there, before we buy.

Apples and oranges.
You have a right to know what is in the product.
You have no right to know what brand of tool was used to harvest it, nor could that be reasonably checked.

Quoting: pbI'm not necessarily saying it should be obligatory to disclose the full toolbox used to make a game, but it would certainly be well received and I hope it will become a good practice.

I agree, disclosure would be nice and IMO beneficial to devs.
But there is no right to that, and some will do it while others won't.