Your suggestion would make lenses better for sure!
I guess that it would be very inconvenient for the manufacturers. There is a lot of in sample variation, which may be hard to explain to customers. I also guess that MTF-testing is not cheap but nor are L-class lenses, so vendors may accept the cost of individual MTF-testing for expensive lens lines.
Delivering individual MTF for each lens would certainly raise the bar.
I see a problem with MTF testing, tough. I feel that the present 10 and 30 LP/mm MTF:s or 10,20 and 40 LP/mm MTF:s may not be appropriate for DSLR as sensor resolutions are closer to 80 LP/mm. Any toughts on that?
Variability amongst lenses of the same model can be significant. We're still in the dark ages I'm afraid. I can see no sound reason whatsoever why each and every lens should not ship with a complete set of MTF charts specific to that lens.
Sorry! I've already just thought of a reason as I write this. There's the obvious reason that most people simply do not care, and that reason is also connected with the reason that most people would probably not understand an MTF chart anyway.
So we're stuck with the problem.
I once bought the Canon 400/4.6 prime and found it to be inferior to my 100-400 IS zoom at 400mm, so returned it for a refund of course.
My own copy of the 100-400 IS at 400mm is noticeably soft at full aperture, no great surprise; sharpest at f8 (no great surprise); and virtually as sharp at f11 as at f8 (also no great surprise).
If you find a 35mm lens which is sharpest at f16 or f22, then you're in trouble.