Because UX people are often completely lost in the fog of their own "brilliance".
Just observe the new task switcher on the Android P preview.
Where before you hit a button (either on-screen on fixed depending on the OEM design) and got a list of previously used apps, now you have to swipe from the center to the edge at the bottom of the screen to flip though them.
Sometimes UX people design the UX to wow the people who (1) control their paycheck (2) review their work externally (3) appreciate UX theory/trends/etc. No different than a programmer using a needlessly complex algorithm to impress people on the internet.
I remember MS word putting in the 'word count' feature for the article authors who reviewed the software. That feature is a high priority item for the writers and people who need their work to fit in a certain constraint, but not for most users.
Just observe the new task switcher on the Android P preview.
Where before you hit a button (either on-screen on fixed depending on the OEM design) and got a list of previously used apps, now you have to swipe from the center to the edge at the bottom of the screen to flip though them.