I don’t recall it being a requirement for General Aviation to monitor the 121.5 MHz International Air Distress (IAD) freq. Our primary UHF radio had a “TR&G” (transmit/receive & guard) selection which would allow us to transmit & receive on whatever UHF frequency we dialed in, and would concurrently receive on 243.0 MHz. I don’t recall that selection on the VHF radio in the Cessna I flew. A second radio could be tuned to 121.5, but many small General Aviation aircraft only have one radio. Point being, 121.5/IAD is unreliable to contact GA aircraft.

OTOH, GA aircraft using 121.5 should be able to contact “most air traffic control towers, FSS services, national air traffic control centers, military air defense and other flight and emergency services, as well as by many commercial aircraft.” See Aircraft emergency frequency on wikiP

If there’s a GA pilot in the house please correct me if I’m mistaken.