I do not know about any official statements, and I agree that this is strangle configured and should be overhauled somehow.
But if you ask for a possible explanation, imho, it stems from the joint copyright. https://en.wikipedia.org/wiki/Joint_authorship
A more mundane explanation would be, this feature is just not implemented and designed well for things like joint authorship and is from a time when Itch did not have over a million projects, but only a few thousand. Having all authors to be admins at least has the benefit, that everyone can configure the page. If the main admin vanishes, the project is bricked. And for volatile joined efforts of amateurs, that is gonna happen a lot in comparison. The professional approach would be to just create a company and have the company be the publisher account. But amateurs do not create companies to publish a jam game.