That itself is false too with a quick look at isthereanydeals showing lot of steam games being sold cheaper outside of the steam store.
Even the Steam key guidelines don't explicitly state that steam keys can't be sold cheaper.
It's OK to run a discount for Steam Keys on different stores at different times as long as you plan to give a comparable offer to Steam customers within a reasonable amount of time.
https://partner.steamgames.com/doc/features/keys
Key word being comparable which is why if you are a user of isthereanydeals or /r/gamedeals you've likely gotten most of your steam games from outside the official Steam store.
I think some people just assume Steam sales must be the cheapest and don't look beyond it.
I am puzzled why people believe Steam keys can't be sold cheaper outside Steam unless they never looked outside the Steam store.
This is one example of a game that isnt too old is Silent Hill F.
https://isthereanydeal.com/game/silent-hill-f/info/
Historical low is $31.49 from Fanatical and Steam low is $41.99
Their approach feels like how lot of companies are currently focusing on AI to market to investors and AI data centers directly, and ignoring what consumers want assuming their opinions are of little relevance. Like how Microsoft doesn't care if people dont want copilot, and keeps talking up the corporate side with the assumption that they know people will use Microsoft no matter what.
Which is much like Epic with them focusing more on giving money to publishers to lock up titles in the past like Final Fantasy 7 Remake from Square Enix over concerning themselves with the demographic of people buying the product.
Its not a consumer focused business model, because the idea of consumers not buying it is impossible to comprehend. Their headlines never seem to be around how its better for the consumer and the benefits to using them over the competition.