A webapp sometimes works fine for phone users. There are things that websites can’t do on mobile. For example, on iOS, only the latest OS version has support for push notifications from PWAs, and even then, they can’t make noise or vibrations. They are always delivered silently. PWAs are also always rendered with WebKit on iOS. WebKit doesn’t support a number of features.
Yes, a mobile app doesn’t help desktop users, but there are waaaaaaaay more mobile users than desktop users.