Convenience Trumps Impulse Capitalism
It turns out that when shoppers are into a pure "Hunter/Gatherer" mode of shopping (ie. get in - get it - and, get out), the impulse nature of decision shopping drops dramatically.
You know what you want and you run to the store to get the items, where do you choose to checkout and get on with your day? The self-checkout station, that's where. A place where there is elbow room, no racks to bump into, and a quick turn-around conclusion to your short shopping experience ... after all, you have better things to do.
This self-checkout thing has become a real problem to retailers, however. Now some of will not even shop in a store unless they offer a self-checkout option!