It is common knowledge that shopping carts are often abandoned by web users. Usability is always secondary. It's never the most important thing about an experience. I will accept poor usability if I get what I need, if the total experience is great. I will reject perfect usability if I am not rewarded with a useful, engaging experience.
Most sites make it easy to fill a cart, but difficult to complete the purchase.
One has to fill in all sorts of information that has nothing to do with the purchase. Then there are often hidden charges, occasionally revealed only at the end of the purchase cycle. Or one might be told the item is not in stock only at the very end. And in many sites, it is difficult to do comparisons of items, so I sometimes use the shopping cart as placeholders even though I have no intention of buying. Put in the three items I am trying to compare so I can remember them.