At some point, automakers settled on a standard stereo plug for their cars, meaning you could install any aftermarket stereo into any car.
Except that they never did standardize. The closest to standardization was probably mid-90's vehicles, when you could at least count on a car maker to have 1 type of connector across all models, and that harness contained standard power and speaker connections. It was still necessary to get the other gender plug for it, and match/connect wires to the aftermarket brand harness included with your aftermarket stereo. It has only degraded from there.
Some aftermarket stereo misadventure examples:
Got a GM vehicle from the last decade? Want that aftermarket radio to turn on/off with the ignition key? Don't want to lose the warning chimes? You need this $100 module and harness that taps into the class 2 serial bus and provides the missing "ignition switched" power wire and speaker for chimes. (Or before that was available, you could buy a bizarre harness that moved your factory radio to the trunk to retain OEM chimes)
Opted for that "premium" factory stereo on that Nissan? No harness available that retains factory amp. You can either cut the amp out and redo all the speaker wiring, or try to be clever and build your own harness with RCA line-level plugs to connect to the factory amp, only to discover that the factory amp inputs are not standard line-level or "speaker level" so you get noise/pops unless you spend another $40 on ground loop isolators.
Or how about a recent vehicle that has modern USB and Bluetooth features on the base stereo? (I'm going to pick on Subaru here) Sure you can still get a harness plug for the basics, but all other connections (overhead mic, USB/aux jacks in console, steering wheel buttons) have no standardization, even between other models from the same maker. (mic has goofy amp circuit on the mic - cannot reuse for aftermarket, USB/aux jack harness - one guy made a business out of custom harnesses like this, steering wheel buttons - need a aftermarket interface module) It quickly turns into several hundred dollars in harnesses, modules, dash mounting kits, etc just to get an aftermarket stereo installed, with difficulty beyond what an average do-it-yourself'er can handle.
Or then there's the stereos integrated with HVAC controls. 3 of the 4 vehicles a considered recently had this. Good luck going aftermarket on those.