In a lot of software I worked with, unbundling core and UI would've been an immense amount of effort. You basically have the option to either maintain two GUIs side-by-side - including integrating new concepts (think, for example, supporting OTP and creating the workflow) and updating both for datastructure changes - or you never update your UX at all. At worst, you end up in a situation where your two GUIs support a distinct set of features.

You can correctly accuse companies of a lot of things, but decoupling the UX really is a very hard problem.