Bad UX is potential security vulnerability. If your system behaves in unreasonable ways, users are much less likely to notice when it behaves in a slightly different unreasonable way, this time because of a spoofing/phishing, etc.
The obvious example: if your system frequently asks for passwords, re-entering passwords becomes a habit (read system one from "thinking fast and slow"), and the user is less likely to use judgement each time they enter the password.
But also, if an OS makes it hard to find all startup applications, allows untrusted code to run in the background without any visible signs, allows terminal code to access all local files by default, etc etc these all can be abused.
One problem is that human psychology is rarely considered as important a factor as it should be by the average security expert. The other is the usual suspect: incentives. The right chain of responsibilities is missing when things go wrong for people because of mistakes that would be avoidable by proper product design.
Regulation should enforce that, but while everyone would benefit from regulation, no one likes the regulation that will regulate the product/services they offer, and the supplier has upper hand when a change in regulation is being considered because they are focused and motivated.