It's because of the British class system. - hear me out before you tell me I'm crazy.
In Britain we have this weird inherited thing that people in shops are somehow a slightly better class than their customers, because they're in business, whereas the customers are just working class scum.
That's why when we go into a shop the shopkeeper ignores you, and generally acts like you're a nuisance, and the customer is like 'I'm really sorry to bother you but I wondered if you'd mind serving me - lowly scum that I am'.
And then they give you totally the wrong thing that you asked for, but you just accept it and pay over the odds for it anyway, because you're so grateful that the snooty person lowered themselves to serve you, and you don't want to make a fuss in case they get angry and won't serve you again.
Shops ands restaurants , exactly the same.
In America (for example) they have no such inherited nonsense. The customer is king. They realise that if they don't serve you they're out of business, and the customer knows it as well, so they expect, demand, and usually get good service and aren't afraid to say so when it isn't forthcoming.