"Eat, drink and be merry, for tomorrow we fry".
Good question! Probably because drive chains require lubrication whereas tracks do not. Lubrication holds contaminents in place where they act like a grinding compound.
There is much more mass in tracks and much looser tolerances. This means dirt gets in more easily, but also falls out more easily. There is much more "room" for wear and loss of dimensions in tracks because of their size than there is in drive chains. Drive chains have smaller parts, made to much greater precision and with much less "room" for wear before they become unserviceable.
Advances in metallurgy may have changed that since WWII, I don't know.