There are lots of high quality photos from the 1930s. Look at studio portraits of movie stars, for example. I’m not sure where the expectation of low quality would come from.
I think most people would have had contact with old pictures from newspaper and books that did not prized image quality so you mostly see bad quality pictures with low dynamic range.
The most important pictures of the olden eras, in an artistic setting, would also be experimental photography which is not necessarily concerned with sharpness and traditional qualities, so you see weird stuff.
And, the main culprit, as for most of society misconceptions. Movies and tv shows, you have to age and crap out a picture to look old. I am certainly that the screwed up videotape effect will skew a lot of the expectation of old footage from the 80s-90s.
Put all of that together, there is where I think the expectation comes from.
I have the book Great War, Photographic Narrative. With images from the first world war, the quality of some of the images is outstanding. Those same images would've look terrible on old mass produced books and newspapers.
It probably comes from all the crappy dim, faded family photos from the 70's and 80's