The reason itch.io is American dollars is because it is following rules about being an American company and following EU/UK tax laws that are set in law. This can change if itch.io has people working in a EU location just like Steam, Nintendo, Microsoft, etc. However itch.io does not have money for such things.
Another thing is that Steam has broken many laws both EU and US. The current new one is about Valve Corp threatening indie developers and lawyers believing Valve corp breaking antitrust laws: https://mybroadband.co.za/news/gaming/396295-steam-allegedly-threatened-to-remov...
https://www.stropse.com/lawsuit-accuses-valve-of-abusing-steam-to-keep-game-pric...
In EU, the same thing: https://www.cnbc.com/2021/01/20/eu-fines-pc-gaming-giant-valve-for-antitrust-pra...
https://www.engadget.com/2019-09-19-french-court-valve-steam-resold.html
the legitimacy of steam following tax laws is up in the air because unlike Nintendo, Microsoft and Playstation who have offices in EU, Valve does not in theory.
also yes someone did try to get information on EU offices: https://steamcommunity.com/discussions/forum/1/617329505861178146
Luxembourg is a well known tax avoidance or evasion: https://en.wikipedia.org/wiki/Soci%C3%A9t%C3%A9_%C3%A0_responsabilit%C3%A9_limit...
https://www.investopedia.com/ask/answers/100115/why-luxembourg-considered-tax-ha...
This is the answer as to why Steam offers lower price, they may or may not be doing something illegal.