This isn't true: Google preloads their design conceptions, namely Material, into Flutter at the ground level, and Material is a terrible set of UI patterns, such as the ones the parent comment was complaining about.
You don't have to use the material design stuff, you can just make your own components, all custom, just as you would on the web. Just replace MaterialApp with WidgetsApp at the root of the application tree.
You don't have to, but it is the default, which means it sets the standard of design there. Unsurprisingly, most things you see made with Flutter tend to either be or be similar to Material.
