I don't know if that's really the case. When I worked retail I mostly really liked chatting with customers.
Now, neither self-checkout nor human register display some discounts, because they're using the same system, and the system applies those discounts at the end. But here's a thing: I can ask about promotions and prices while the cashier is scanning. Also, if I realize after paying that something is wrong, I can immediately dispute it with the cashier, instead of waiting for the employee to maybe come in the next 5 minutes (they're probably on the other side of the store, busy unloading some boxes).
As for the pressure, I personally feel much more pressure coming from an unyielding machine than from a human.
I have to add that I have basically always had poor experiences using the garbage web-app menus that restaurants use. It is also much slower to get an overview on menu items on a small phone screen.
The main way they have of encouraging you to use self-checkout is just having way too few cashiers.
I think you're right about some people. But it's true of any job. Some people are pleasant in that job and some people are the opposite. Just because you don't scan boxes of diapers for 8 hours doesn't mean you're immune from this behavior.
I think it's just a matter of treatment, frankly. Many cashier-like workers are treated like garbage. Imagine instead if the grocery store cashier had a pleasant or even funny conversation with everyone in their line over the day - I think it would measurably improve their mood.