In the olden days, you would be expected to use the UI toolkit provided by the OS, which was designed to provide a consistent and complete implementation of each GUI element, which not only made things easier to navigate for users but also provided things like customizable theming and accessibility features more or less 'for free'. Nowadays it's much more like a free-for-all, and every application is just a little bit different, sometimes on purpose, sometimes just by accident. Accessibility is a complete crapshoot.
The web is similar. The people pushing for using 'the platform' are trying to capture exactly the same set of advantages as the old-school OS UIs, as well as the additional complication that we have a much more diverse set of devices with UIs nowadays.