As an old timer who custom-ordered my first car in 1970 (Plymouth GTX), I remember going through a multiple-page list of options available. You could build one to your specs and avoid paying for stuff you didn't want. That changed decades ago with increased robot usage and automated assembly lines. There are still cars you can "build" like that (Corvettes come to mind) but they aren't what you'd call "mass produced" cars.
My own theory, aside from simplifying the process, is that it's a profit incentive. (I have no proof of this, JMHO.) Repair shops today are mostly R&R shops. They don't "repair" anything, they just replace stuff. We used to rebuild starters, alternators, transmissions, brake cylinders, etc. Today those components are less likely to ever need replacing. There is also a lot less maintenance. Where we used to make money on brake jobs, hoses, belts, lube jobs, transmission services and such, today you can go a couple hundred thousand miles before needing any of that. Shops have to make money doing something else, so they deal with the complexity of what is essentially a rolling computer. The factory pays for replacement which is usually fairly simple, the parts department makes a bit of profit on the parts, and only those of us who keep cars beyond warranty periods get hosed on maintenance.
So what we see today on used car lots are cars with toys that are too expensive to replace -- like heated steering wheels, damaged park and blind spot assists in bent bumpers, rear view mirrors burned out with a careless button press after an ice storm. You really have to think about what these toys cost to maintain after assembly and whether or not the car is worth the repair.