Many more people could read music 100 years ago, and sheet music sales were high. They would hum a tune directly from the page, and if they liked it play it, usually on a piano.
The wandering musician filled the place of the record player, and so people still hummed tunes that were popular.
In my own case, the tunes that fill my head are mainly the great themes written by Beethoven, Brahms et al. In times of stress, I put on a pair of headphones and sit up all night listening non-stop to great symphonies. The music drives the stress out of my mind and I eventually fall asleep in the chair, de-stressed.