Note that there's a lot of hardware on that list that supports OpenGL 4.5 and indeed has drivers for that on other platforms but on OS X all top out at 4.1. That may not sound like a big deal, but OpenGL 4.3 brought full compatibility with GLES 3.0. That's a biggie if you're, say, an indie game dev that wants to do a cross-platform game that can span from mobile to desktop. And 4.5 brings the robustness support that allows for secure WebGL.
Just look at the supported version list: https://support.apple.com/en-us/HT202823
Note that there's a lot of hardware on that list that supports OpenGL 4.5 and indeed has drivers for that on other platforms but on OS X all top out at 4.1. That may not sound like a big deal, but OpenGL 4.3 brought full compatibility with GLES 3.0. That's a biggie if you're, say, an indie game dev that wants to do a cross-platform game that can span from mobile to desktop. And 4.5 brings the robustness support that allows for secure WebGL.