Or the process ensures that they all perform well of course.
The JFET noise is a matter of device geometry (defining the flat noise) and process (defining mostly the corner frequency, also the x in 1/f^x), much simpler than in a bipolar device. There is very little room to wiggle in the variables, so the decision to avoid measuring the noise of individual parts is perfectly justified. In a mature manufacturing process, it would be very unlikely to get a part all in the datasheet guaranteed limits, but with an outstanding high noise.
Guaranteeing certain parameters "by design" is a very common practice in the semi industry today. Such parameters are usually mentioned in the data sheet, either explicitly "guaranteed by design", or by mentioning only typical values. Of course, for the right price and quantity, the noise could be 100% tested and guaranteed, but then you would not enjoy the prices .