Because they make money by selling telescopes to people, and there is a large enough number that will only want the highest strehl.
Then they are stuck with a pile of scopes they can't sell.
There's a common measurement technique in psychophysiology called the just noticeable difference. It's the minimal physical change in a stimulus that can be distinguished 50% of the time (i.e., the threshold of chance).
It's useful for experiments where you either want to understand the nature of human perception of physical quantities like light and sound, but it's also useful for characterizing stimuli for new experiments--I do experiments with voice pitch, and it would be daft to present stimuli inside the JND if you want to see how people react to different pitches, for example.
I doubt anyone has ever attempted to express a JND in terms of Strehl in a quantifiable manner. The closest I have heard is some crude comparisons to see if people in the filed can distinguish 1/4 from 1/10 wave (or something like that) mirrors. I think the answer is that most people cannot.
In any case, this experiment could be done, and we could find out the JND of Strehl for the average amateur astronomer, and characterize 'high' and 'low' in that fashion.
I somehow doubt it would catch on, but it would be interesting to find out what the number is. It would also be interesting to find out how variable individuals are in the JND--age eyesight, experience all being relevant moderators.