I recall seeing a discussion on this a while ago, with the comparison made to the Swing LAF. Basically, this idea of themable high-level components sounds good in theory, but in practice is not able to provide the level of fidelity needed to make a UI actually look like the platform’s native UI.
While it seems like one could just adapt the button, text field, and other component styles, it’s worth remembering that the layout and design thinking is quite different across platforms too. For example, on Android one might use a checkbox where iOS uses a switch. Or iOS using a back button in the header where Android might omit it and rely on the hardware back button/gesture.