My theory is - calculators. when we did things by slide rule, in our heads or by hand calculation we had better sense of the "correctness" of the results. So many little things have been forgotten that we learned from day to day work. For example, in these bearing housings we're talking about, there is a nut that holds the bearing, and this nut has a set screw. The originals have a bit of lead in the hole that prevents the set screw from buggering up the threads, and not one of the "rebuilds" I've taken apart seems to have this little piece of lead..