The thing is that many of the older designs still turn up in new devices,so teaching someone how to use a 8255 is still a useful exercise.Chip designers are no more inclined to re-invent the wheel than anyone else.I expect that things like uart code has changed very little over the years.
We had a new PCI bridge for the strongarm from digital recently and it was a bastard to use as it was rewritten from scratch and was virtually a beta release.