Keep in mind that Mesa actually implements most of OpenGL for you. Its not like you are implementing a whole OpenGL driver from scratch, you are mostly implementing a hardware abstraction layer.
My understanding is that this hardware abstraction layer for mesa is way easier to implement than a full vulkan driver, especially since the earlier versions of OpenGL only require a small subset of the features that a vulkan driver requires.
My understanding is that this hardware abstraction layer for mesa is way easier to implement than a full vulkan driver, especially since the earlier versions of OpenGL only require a small subset of the features that a vulkan driver requires.