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.
true, and that saves you having to write the frontend in html/css/js, but you still need to write an api-only web app to act as your backend. the "going with the grain" path for a desktop app is to be running on a local machine, with all data stored locally, and interacting with the user over a keyboard and mouse, not the network.
you can absolutely write a desktop app as a web app client, but it's a lot less well-lit a path than either writing a standalone desktop app, or writing a full-stack web app.