Because Apple has had it's head up it's arse for years when it comes to making the OS functional...seems they hired a bunch of Windows people so they can make is 'simple'
Maybe some day they'll refocus on their core products instead of their stupid phones.
Windows has sub-pixel anti-aliasing.
So, why did Mac OS X remove it? I'm no expert, but I've been reading some comments out there from developers, and they say from a purely design point of view, in the world of Retina screens it makes logical sense not to use it. Sub-pixel anti-aliasing offers no real advantage for Retina screens. For non-Retina screens, it's a big advantage, but it's also a real PITA to implement, and such implementations can cause many problems if not done completely correctly. So from an OS design perspective where the future is all Retina, it makes sense not to have sub-pixel anti-aliasing at all.
https://news.ycombinator.com/item?id=17477526
But of course, Apple STILL sells non-Retina Macs, and people buy non-Retina screen to use with their Macs too.
Maybe once 10.15 comes out I'll invest in an external 5K, screen. I have a 2010 iMac 27" I'm using as an external monitor for my 2017 Retina iMac 27". The 2010 2.5K screen looked bad enough already, but without sub-pixel anti-aliasing, it looks even worse, esp. sitting next to that Retina iMac 2017. I've turned sub-pixel anti-aliasing back on, but even then, the difference in text quality is huge. Plus, with a 2.5K screen, I can't use non-native screen resolutions. If I got a external 5K screen, I would be able to change the screen resolution on both screens at will. I also suspect a later iteration of macOS (10.15 or 10.16?) will no longer support turning on sub-pixel anti-aliasing.