There also is a lot more to document. That Marantz receiver or shortwave radio probably had less than 100 transistors. Your smartphone has billions. The cost of providing documentation alone may be sufficient reason not to provide it to customers (yes, they have documentation, but that may not be sufficiently checked for use in a B2C setting, where you likely can’t provide documentation with a “reality may be different” disclaimer)

I also think that, historically, electronics were documented because they broke down so often.

You just had to be able to spot a broken tube in your radio, clean a potentiometer, etc.

The IP I'd say is the core of the problem.

There are only so many way ways to skin a cat (in nuts and bolts engineering). When constrained by physics, everyone is going to converge on the same fundamental designs. You therefore have two or three options. Take or leave the licensing agreement of the IP holder, which need not be reasonable or effective; say screw it, use the IP and hide the schemata behind enough trade secret bs red tape that no one is comfortable leaking it, or give up.

I wager the second option contributes to things when number 1 fails. At least, it's the only reason that holds water for me, then again, I haven't gotten into that industry enough to vouch for the presence of skeletal remains in the closet.

Note that these features exist on the M1.