They are designed with Loss Prevention in mind, so they end up treating every customer that walks up to them as a criminal looking to steal something.
This is one of the reason I still use a human checker when I go to the grocery, it is far more efficient and takes less time than the Self Checkout.
Now if they ever perfect what Walmart has been working on, either the "Check out as you shop" system where you scan items with your smart phone as you shop, or the RFID based system where you push your entire cart into a large RFID reader and it scans everything in a matter of seconds... that might get me to use those systems