An application team will probably have a UI design resource - either their own or shared with similar team(s), and that design team will dictate how things look and work for that application team. The design team will have to justify their decisions to their own (design) boss, and upwards all the way to the top, if with decreasing resolution.

Design at Apple isn't just about how it looks, the design teams are tasked with how it works too, and I'm not saying they don't do that job well - there are usually a lot more corner-cases that have been considered than what some random blog talks about. Sometimes the "why don't they do it this way" question is simply because the asker hasn't thought about the implications on the scale that Apple does. Other times, it's a mistake/bug/oversight.

But the idea that something as major as the glass look and feel wasn't authorised at the very top and mandated to all the teams down the pyramid is just naive, at least if you have ever worked at Apple.