OS X Leopard is very old in Apple terms, released in 2007 and being the last release to support PowerPC processors. (that even pre-dated the functions that you say above being standardised, they were just GNU extensions before)
For Vulkan, MoltenVK works very well and is officially supported by Khronos directly. (Metal is a much more approachable API than Vulkan, but oh well...)
Okay, 2007 is pretty old, I can't fault them for not supporting these functions. However, this is a noticable trend with macOS. Their pace with modern Unix is severely behind, and I often get bug reports from macOS users lacking some API that's been available on open source Unicies for 5+ years. Plus, even if Linux in 2007 didn't support what you need, it's open source so backporting the required functionality is feasible, and impossible on macOS.
MoltenVK is a good workaround, but open standards > workarounds.
For Vulkan, MoltenVK works very well and is officially supported by Khronos directly. (Metal is a much more approachable API than Vulkan, but oh well...)