"WTH is so hard"? Back then, every car's instrument panel was different and NOT flat. Curves, angles, peaks etc in the middle of the instrument panel area where the radio was. With a specific-to-the-car shaft spacing, usually, too. NOTHING universal, which is what all of the later "modern" radios are. NO "fit kits" to adapt them, either. ONLY upgrades from the factory were to AM-FM mono until the 1968 model year when 8-track tapes became optional, which required a different bezel, too. THAT is what is so hard!
Wanted FM, an under-dash unit to "adapt" to the in-dash unit. If you wanted a Stereo 8-track and FM Stereo addition, that was in an under-dash (or hump mount, in the case of Pioneer) unit. THAT was how it was done back then. Of course, had to add the stereo speakers. Usually in the front kick panels or cut-up the door trim panels for some 5.25" round speakers, plus the dual 6x9s in the rear package tray deck.
Back then, no "head units" or need for "amps" as sub-woofers hadn't been invented yet.
A different world back then, as to "auto sound".
Respectfully,
CBODY67