The easiest way to share files on a desktop computer is for everyone to use the same login, and leave all the files on the desktop or in the "Documents" folder.
On the other hand, a desktop can be a personal thing. If I put a file somewhere, I like to know it'll still be in that spot when I get back. Computers are tricky enough. We don't have to add the work of other unpredictable humans to make them hard to use.
Once a household starts using multiple logins, you run into a completely different problem: how do you share things? I took all those pictures and my daughter wants to see them. We took turns typing in Xmas presents as we opened them, now where do we put the list so everyone knows what Thank You notes to write?