because to most people, trying to find the magazine would be at best, difficult
you can't just presume everything is easy. Try to find a copy of this specific issue this week, and let me know if you were successful.
it's also possible the people who owned it may not have even known what it was. Maybe they found it at a flea market 40 years ago
hey, here's an exercise.. go to Heritage and look up Frazetta paintings and examine each one against the original printed material.
Frazetta changed many of his paintings after publication. The cover to Vampirella #1 - he removed her clothes and she is now, naked. Kevin Eastman owns this piece
look for a copy of the Monster Mania from 1966 with the cavemen cover and match it against the painting which I believe Heritage has sold.. no longer the same. he changed the #@*&% painting! Guess what - artists do this all the time! Not that it has anything to do with the Valentino, but examining them will add to your education
I spent years selling this type of art.