They may pick them too soon in order to ship them hard back east or maybe they gas the fruit or something and it isn't alive anymore. We always used ones that we picked up off the ground or off a tree.
It's funny, when I was in college and really trying to make it by myself, in other words I was pretty poor and not spending much money on food. Anyway, avocados were free and plentiful so I ate a lot of them. Avocado and brown rice, avocado and cottage cheese, night after night. After I had some money and could buy real food I didn't eat avocado for years, maybe 10 years. I also wouldn't eat anything with lemons or drink lemonade for awhile after I spent one summer working at the lemon packing house, really gross.