Azzanation said:

Why is it so hard to add a shopping cart?

There might be things you don't even think when using a shopping but you still have to take them into account when making one. I don't know if there are though, and even if it is hard, I doubt it's that hard. It probably doesn't even matter though, since this is most likely a matter of priorization: Epic has determined that some other features are more important, and has thus decided to implement them first. Epic seems to have a lots of resources to pour into developing the store, but there's still not enough workforce to work on all at once. At a quick glance, it seems that not prioritizing the shopping cart feature to be ready before the first major sale is a mistake.