Because somehow, genetically, the two traits of reproductive cycle and resin production combined into one co-dependent trait? Ha ha! Each one is a separate trait, just like taste, smell, color, height, etc, that can be bred into or out of a strain through subsequent generations. Ha ha ha!
I've grown many autos as an experiment this year and they all vary in potency just like photo strains. They are exactly the same except that they flower in response to age of the plant rather than the light cycle. They use the same soil, same nutes, same ph, etc. I will say that in general they are smaller and therefore the yields are smaller. They also are sensitive to abuse or topping, as there is no recovery period or the ability to extend the veg period.
So just like anything they have strengths and weaknesses you need to research then judge for yourself if autos are appropriate for your garden. Certainly grow them and try them before you toss out a blanket statement about resin production, potency, and other characteristics. I'm smoking Paradise's Pandora right now and it is fruity, spicy, ooey gooey two-hit-quit gear my friends ask for when I break out the head stash.
Happy growing!