Even the SPEAKERS inside of the Model 3 and Y are designed by Tesla (and manufactured by a vendor).
That's the big difference. It's also the reason why legacy manufacturers will struggle to effectively respond to Tesla. It takes a LOT of money and egos to upend generations of culture (and accept hundreds of millions in losses in the process)
In this particular case, is this really a good thing? Porsche's high end option for example is getting the speakers, amplifiers, tuned filters etc. from Burmester. Since Burmester specializes on this kind of thing, and the sound system (not the head unit and interface) are pretty much entirely distinct from the rest of the car, except that it has to be tuned to the environment it will exist in, that intuitively feels better conducive to focus areas and quality all around.
I think the German manufacturers just understand their customers vanity, sadly, where being seen to have the brand matters. The label is practically more important than the sound beyond a certain point.
But you may have a point anyway: The Bose system in the same car (a vastly more popular option because it is much, much cheaper) sounds worse than the cheap sound system in my old Ford.
https://www.suncoastparts.com/product/SKUBERTWEET.html https://www.suncoastparts.com/product/SKU680TWEET.html
Source: https://twitter.com/PaulMaric/status/1160777733825150976