The reason is that most manufacturers, including those who publish measurements, do not measure at the platter and probably do not know how. What is widely done and extremely misleading, is to report the accuracy of the chip used. This information is provided by the manufacturer of the chip and really has no correlation with the turntable's actual speed accuracy.
Turntable speed measurements for speed accuracy?
I'm curious why don't turntable manufacturers publish speed accuracy measurements? Some do, but most don't.