I think it’s just a consequence of the computer market maturing. I’m sure if you bought a car in the 1900s you got detailed schematics as well
And if you buy anything nowadays you will get undocumented locked-down hardware. It doesn't happen if it's recently invented or existed for centuries.
Compare 1980's sound systems to today's sound systems. Or 1980's fridges to today's fridges. Or washing machines, circuit breakers, whatever.