How does Arduino IDE for OSX work without drivers?
Windows 8.1 is the last version of any common desktop system that requires CDC/ACM (that is what the Arduino's USB port is) drivers.

OS X is based on BSD which, like Linux, has had CDC/ACM drivers built in since (pretty much) the dawn of time. It is only really Windows that has had the concept of drivers for anything that isn't vendor specific and/or complex (such as video cards, or devices that don't adhere to published standards).

Even Windows 10 now has the drivers for CDC/ACM built in (it's only taken Microsoft 20 years or so to catch up with the rest of the OS world).