Here's a little insight from someone who's been in it almost since the beginning having built my first kit around 1958 about why those AMT kits, some of which are from tools that are over 40 years old, are the way they are and why your comparison is veru unfair.
The newer kits are tooled for an entirely different purpose than the older kits were. In fact, the older kits didn't start out as kits at all. When the new cars came out, the auto manufacturers would contract with companies like AMT to produce what were refered to as "promos", short for promotional models, that were used in showcases in the dealership to let the prospective buyer see what the cars looked like in the different colors offered without having to stock all of them in the full sized cars at all times.
Sometimes the dealer would give out these cars to his better customers, especially those with young boys, as a sales premium or appreciation or even to help "seal the deal". The dealers needed models that were ready to hand out and not kits so they manufacturers, after getting all the info on the new models, would make a simplified replica with as few plarts as possible to produce an accurate miniature of the full size car for the car maker's dealer showrooms.
That's why the interior consisted only of a "tub" with all the seats, a dash and a steering wheel so that the "kits" could be built by AMT and others in a very short time so as to get the new little cars in the showrooms when the real one hit. The full size car makers paid for the tooling and the buildups and the model companies made a modest profit. Along about '58 it got really interesting as far as us modelers were concerned.
AMT started doing the promos, and with the orders from the carmakers there were hardtops, convertibles, station wagons, four doors and pickup trucks, and after the orders for the dealers were fullfilled they added some tooling for a few custom parts, printed some decals, put it all, unassembled, in a box and our auto modeling hobby was born from these humble beginnings.
At first there were no engines because a promo's hood didn't open but later the kit makers added engines with stands and later opened the hoods in the retooling process after the promos were finished. That's why there were so many different body styles kitted back then and why they weren't as detailed as the kits made thirty or forty years later.
Before you go condemning a kit for its lack of detail stop and think how long ago and for what purpose the tooling was cut. Would you rather have a simplified kit of no kit at all of that subject because there isn't enough of a customer base to support a new modern tool?
And another thing about those simple little kits that today's "sophisticated" modeler convienitely overlooks is that those kits got built! And not only by the "master modelers" that gripe about the vintage kits today, but also by the kid brothers of those "kit assemblers" because they wanted to be like big brother and that grew another generation of buyers that made the next batch of kits possible.
And the most important thing about those simple kits was that when you got it built, it looked like the vehicle you were modeling because the shape of the body and the overall "look" was built into it to please the people that built the real car. Beginners built them out of the box on their way to becoming modelers and modelers used them as the basis for building their "visions", from customs to racers to whatever their imagination could concieve. And that. my friends, is what modeling is supposed to be about.