well thats not entirely true either Mac, barbers, butlers, and slaves usually shaved people, until world war one I don't think many men shaved themselves at all, the army and the availability of the DE was the first time most men began to shave themselves, in large numbers anyway, so a lot of men had to figure it out on there own then as well
My point really is that, whether they were shaving themselves or not, those people came into shaving with the expectation that it was something that they had to pay attention to -- that it was a skill they had to learn. Modern cart razors, on the other hand, have made it so that really the only thing you have to pay attention to is not missing a spot.