I got involved in audio in 1971 when I bought my first system: Sherwood 7100 receiver (there was no remotes to my knowledge yet for receivers); Garrard SL95B turntable with some level of Shure cartridge. A pair of Rectilinear 11 bookshelf speakers..for 1971, this was specialized, if you do not recognize this! not everyone was going to the audio store to get this sort of equipment to listen to music.
Audio was always specialized - my vintage Yamaha integrated amp cost $800 in 1977! how many could afford that back then?
what's wrong with your way of thinking is that you're comparing the complexity of today's equipment with that of 1971's. That's an apples to oranges comparison. The gear of 1971 was specialized for 1971 & used whatever best level of integration (discrete or IC) that was the SOTA at that time. Looking back, it looks like that gear was simplistic.
And, the evolution of audio is like any other industry. For example, in 1903 you can see what the Wright brothers' aeroplane was & today you know the complex F16/F18/F22 fighter jets the USAF flies. Another example, look at Ford's Model T back then & then look at today's complex Lamboghini/Ferrari, etc, etc.
So, my counter question to you is: Why has the aircraft industry & the automobile industry become so specilized over the years?
IOW, use you mind to think a little about the subject rather than just posting a question. A thinking will not hurt you (unless you are one of the 3 stooges who said "I think but nothing happens!").