Earlier quoted context omitted.
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 yo…
Desktop apps can use HTTP(/grpc/rest/graphql) for communication just fine, that's not really problem.
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.