Nefore better UI and UX help you beat SAP and co, you need compliant, and industry specific ERP functions on par with them. And then you need to sway all those existing customers away from them. In market that doesn't value UI half as much as you seem to think.
I agree that the enterprise market does not value usability very much, and I would even agree that the enterprise vendors made the right choice to deliver anything at all, in the early stages of ERP (even the first two decades).
None of that means the software is good: we can recognize the smart business choices those companies made and also abhor their products.
They provide what Alan Cooper calls "Dancing bearware" - it doesn't matter how badly the bear dances, they're just grateful to have a bear that can do it at all.
