Yes the filesystem is from the 80's and from a period where Unix filesystem model had perhaps not gained the dominance that it did in later years. The question is why is it still being used in OSX 15 years after a complete redesign of the system around a Unix kernel?
They had to support HFS+ in order to convince existing users to upgrade. The first five releases of OS X had something called "Classic mode" which was basically an instance of OS 9 running as a process, so that people could continue using their pre-OS X apps. It took six years before enough developers had upgraded that Apple could stop supporting Classic. And Classic, of course, was all about HFS+. So Apple could either backport this hypothetical unix-style filesystem to OS 9, do something kludgerous with single-machine file-sharing, require all users to partition their drives and keep everything they wanted to use in Classic on the HFS+ partition (a tough sell!), or just use HFS+ and get on with it. They wanted to stay afloat, so they stuck with HFS+, and here we are today.
