There is no reason, really. People just don't think about using game engines to write applications. This is part of why Flutter instead turned this around, maki

There is no reason, really. People just don't think about using game engines to write applications. This is part of why Flutter instead turned this around, making apps act more like they're in a game engine, with the Skia renderer. Cross platform in this case is just using the renderer for each platform and it works super well. If you like Godot, therefore, you should definitely check out Flutter as it's custom built for writing apps.