First of all, any single shopping cart is typically accessed by one single process at a time: the user's browser. I can of course open two or more browser sessions, but this is not a typical use case, it definitely has an upper limit (how many browser tabs can I use concurrently?) and most importantly, lost or duplicated transactions are easily spotted and corrected (you do get plenty of chances to revise your order before paying).

A bank account can be "hit" by different sources at the same time (just think of a company bank account, not just a personal one) and most importantly, duplicated or lost transactions are much harder to prove and rectify.