Great question by the OP. I ask this myself.
I learned the rudiments of bicycles from my mother (former land girl and very practical). Other stuff I picked up as I went along.
I picked up the rudiments of motorcycles and cars from my father, my big brother and friends.
It was easier then. Both car and motorcycle could be patched up with parts from breakers. I could go 'into the yard' at my local breaker and clamber to the car on top of a pile to bag an unused spare wheel. I could spend twenty minutes upside down pulling a starter or an alternator/dynamo out of a wreck. Things really are more expensive these days, not just because of inflation.
In terms of bicycles, I see that there is no peer reference for my children. Their pals may ride, but do not get shown by dad or mum how to change a BB or tension a gear cable or set the brakes up. Mine do, but I find I am starting from a position of almost complete mechanical ignorance. This is my fault, but there is no way I was going to make them change tubes and headsets at three years of age....
It's not all doom and gloom, but the need for specialist tools doesn't help - and let's be honest... there are a few.
The price of parts is also a turn-off. In truth, most people will be just as fast over 10 or 25 miles whether they are on Veloce or Super Record.... But somehow the latter is more sexy. For the retailer, it probably is.
I no longer work on car engines (absence of diagnostic equipment) but still do brakes, exhausts and so on.
As the OP says, bicycles are extraordinarily simple. Carbon brings some concerns about correct torques, but I still do it by feel.
I'm sure there is a benefit to some folk in mystifying the whole 'bike tech' thing - and I am still a bit vague about trueing wheels.... but most stuf is just fit and forget these days. Even a 20-year-old bicycle seems quaint today, with its BB made of separate components....
Hurrah for this thread and if you're interested in the working, start mucking around today.