Agree.
I believe one the specs for an outdoor-rated extension cord is that it must be able to withstand being run over by a typical car when laid across concrete. (I should check on this.) Hence the thick PVC jacket. The jacket material is also formulated to be moisture-resistant and UV-resistant, and is constructed to withstand a certain amount of tension. And as you mentioned, 12/2 NM-B (“Romex”) consists of solid conductors. It is not flexible and could break due to fatigue.