I don’t know how common it is for hams to collect these but my father has thousands, so I’ll try to get him to send what he has your way

Extremely common.

Before internet the physical cards ware the proof that you had had that contact. They ware needed for awards in in many countries to even advance from your license class upwards.

In Finland one had to have 300 confirmed (paper QSL card received) contacts with morse code before you could even attempt the tests for higher license classes that allowed voice and data. Thankfully now abolished, along with the mandatory morse code requirement.