Quote - I’m sure Leonardo did not give formal credit to the model who was the basis for the Mona Lisa, so Leonardo must have lacked some artistic ethics – according to your post.

Actually, as Vasari (writing 30 years after Leonardo's death) states that she was the wife of a Florentine businessman called  Francesco del Giocondo there's a good chance that it was well known who she was when Leonardo painted her. Anyway, I get your point.

I find this reluctance to credit others surprising. I don't regard it as a 'silly practice' for much the same reasons as Eric gives above.