Too many developers nowadays don't know this. On any HN discussion of UIs, I've been noticing a growing number of younger devs insisting that usability is entirely subjective (their words, not mine). It's not just that they don't know about cleverly thought-out things such as safe triangles in nested menus or all the affordances/signifiers espoused by Don Norman et al. The bigger problem is that they don't know what they don't know, and they come across as being unwilling to learn.

It does make UX discussions frustrating and meaningless when they could, and should, be interesting and a learning experience for us all.