Hi illusion513! I have thought about the same thing and came to your same conclusion. These are all unresearched points on my part, but...
I think Apple Watch owners represent, on average, a more technology "invested" subset of Apple product owners...and a more affluent subset.
Such people tend to upgrade their phones more often and so there is not a lot of iPhones 6s and 6s+ owners who own CELLULAR enabled Apple Watches.
iPhone SE owners are outliers with regard to the above because they do not have an upgraded version of their small form factor iPhone to buy. So they have held onto their SE's like an alligator holds onto its prey.
Who knows what the real reason is, but I don't see how it could be the hardware or software as the two devices, according to everything I have read, are the same device except for their screens, cases, and battery size.
With the highly rumored release of the successor to the SE to be released as soon as March, it will be interesting to see if the number of Apple Watch complaints drops precipitously because these original SE owners will buy the next generation model with modern internals that seem to have no problems with call functionality.
I hope Apple does not consider that a way to solve the problem as I am a larger screen fan and am waiting for the release of the iPhone 12 to finally bid my 6s+ a tender adieu.
Kind regards, Mr. Luigi