they could, but you would have to do pretty much everything from scratch - set up a server, figure out client-server communication, handle state changes, flaky networks, etc. with a webapp you get a lot of that infrastructure "for free" insofar as the entire stack is designed around the multiuser experience, and you are going with the grain of the platform. not to mention all the frameworks and libraries that help you with most of the common tasks you will face. you can follow a rails tutorial and get a small database with a multi-user read/write user interface working within a day or two.