You don't have to override the native styles of UI controls with CSS. If you don't add any CSS, they will look just like in native apps. It's a choice, just like you could implement your completely custom controls in native apps if you wanted to.
Sort of: it’s possible for a browser to render the controls with native components. Designers of web applications tend to think their aesthetic preferences are more important than the preferences of their users; and, browsers have not always used real native controls: Google Chrome just shipped its own controls for form inputs, for example. Historically, various parts of the Cocoa text input system also worked incorrectly in Firefox and Chrome.
