Largely due to support in Unity/Unreal. They would be using OpenGL ES still if they had their own engines except maybe the biggest game studios that always have custom game engines/rendering integration.
So lumping together all of these variants and calling them "most used OS" is not useful in any sense. They are very different for users, and very different for devs.
This is specially relevant when you want to develop a game against a graphics API. Samsung has Vulkan on their flagships for a couple years now, but not the mid/low-end devices. Pixel supports Vulkan, but the units sold are so low that they are a rounding error. No other major manufacturer significantly supports Vulkan either. In this and similar scenarios, the framework used by ios may well be the "most used" mobile framework.
Then you're correct by your definition, but not by most people's definition.
And even then, it is only required on those that support VR.
Metal is supported on across all iOS devices still getting updates.
Since nearly everyone's GLES 3 hardware both supports vulkan & has vulkan drivers already there's no particular reason to believe that an OEM will decide to just not ship vulkan support. It's more work for them to remove Vulkan from the SoC's base image than it is to just not touch it.
The low-end GLES 2 hardware is the only problem, which is a market iOS simply doesn't exist in at all. So if you're cool with ignoring that market, go for it.
https://godotengine.org/article/abandoning-gles3-vulkan-and-...
Lets see if they have better luck with Vulkan.
[citation needed]
Not that they have their own API, but that it's at all better or faster than Vulkan. Remember Vulkan primarily came from AMD's Mantle. It's not the creation of a committee catering to the lowest common denominator.
And no, it's not only CAD still stuck on OpenGL. Games use it, too. Most of mobile is also on OpenGL ES. Increasingly more of them use it than they used to, even, as increasingly games are using off-the-shelf engines that are already cross-platform and have an OpenGL back-end.
The single largest consumer OS in the world (Android) only supports OpenGL ES & Vulkan, even, with no proprietary graphics API of any kind.