The comedic value of the IBM PC architecture and its myriad of cautionary tales cannot be overemphasised. From the keyboard controller controlling a short to the A20 line to the rather questionable choice of using a programmable timer to generate audio (BTW, what was used in the cassette port?) and the 6845 sync frequencies that would ruin (if not set fire to) an MDA monitor, all the way to write-only EGA registers and the TSRs that were needed to keep track of which video mode the screen was on, the list and its hilarity, is enormous.
I assume the IBM engineers who came up with these ideas had a lot of fun.
IBM is incompatible with fun
It was about cost mainly and backwards compatibility
Maybe officially, but it's hard not to appreciate engineers getting away making the most idiotic, borderline surreal, technical decisions for multiple generations of a machine architecture. The PC must have been the most hated piece of hardware ever to be designed within IBM and the outlet of countless corporate-induced frustrations.
Making something correct with no regard for cost is relatively easy. Making something that works at all but costs less than $5 in bulk can be a lot more difficult.
I cannot well repeat how there I entered,
So full was I of slumber at the moment
In which I had abandoned the true way.
http://genius.com/Dante-alighieri-inferno-canto-1-annotated/
My point is, it's easy to get into this kind of weird design world, if you just do the quickest thing at every step, with no ability to think about how the big picture is going to look.
I'm sure nobody wanted to make a weird architecture, because they're harder to extend and harder to write software for, but if you always have the deadlines breathing down your neck, don't be surprised if you end up in a scary forest indeed.