Quite simple, they didn't pixel peep obsessively.
If you were to post work from those days online, they would be sneered at by many for their poor quality. A better qaulity image, does not necessarily make for a better photograph. Many times it's better to have a grainey not perfectly sharp image than no image. Look at iconic images from the last century and many are not sharp, grainy, blurry or all 3 in the case of some war photographs.
I think jjj makes an excellent point. I visited the (US) National Gallery of Art this weekend to see the show on Robert Frank's The Americans
, and was struck by the fact that while many of the images were what we would consider technically flawed (imperfect focus, grainy, signs of motion blur), that did not detract from their emotional content, and in some cases even enhanced it. While I love being able to get (technically) better images with my DSLR than I could get with medium format 20 years ago, a sharp image of a fuzzy idea (to quote a famous photog) is still a mediocre shot.
[Just my two cents - NOT trying to steer the thread toward "Your camera does/does not matter! " ]