How do those other platforms handle the merchant situation? Who acts as merchant? Itch does not act as merchant for all projects.
While not a technical block to implement a feature, itch is low cost. That prohibits implementation of some features. If only because it would cost money to implement them. Just think about direct messages. Why are there no direct messages on itch? It is a thing most run of the mill out of the box message boards have. There is no technical barrier. My assumption is, it would cost money in terms of staff time for moderation and arbitration, spam removal and so on.
My best guess is, they shy away of implementing such a feature for a combination of reasons. Among those reasons, that for games mostly unfinished projects would try to set out like this and not already finished projects would try to start selling like this. And fledgling collab projects of amateur developers from around the world sound nice, but are bound to bring in a heap of work in support. Mostly unpaid support with the current pricing scheme.
Or let me ask differently, is there a way to do such a revenue sharing on Steam? Maybe on some other game platform? I got the impression with very few data points, that those revenue sharing things are more a thing of basically book publishers. Books are only a very small part of itch.