Your premise is incorrect. It is obviously feasible...just look at the layout of every mall in the US. The question is, why haven't they done it?
I fully expected to see such attacks on Black Friday 2001. It would have sent our economy into a bigger tailspin than the one we actually experienced. The only reason I can surmise for it not happening is that the jihadis don't really understand us and don't realize how much this would affect us.
I hope we are not giving them any ideas they didn’t already have.