Continuity mistake: When the bank teller counts out the $800 near the start, from her point of view the bills are all in a very neat pile, but from the other angle they're slightly splayed out.
Continuity mistake: When Ricky Gervais drives home to the big crowd outside his house, he unbuckles his seat belt and it slides up and along the inside of the car door. In the next shot when he opens the car door, we can see that he's coming out of a different car, with no seat belt visible inside the door. (00:51:25)
Continuity mistake: When Anna and Mark go out on their date, in Anna's drink the straw is first sticking out pointing to the right, but in the next shot or two, it points to the left, and then back to the right one shot later.
Answer: It's an imaginary world with its own logic. You could theorize that people were raised this way. Also, you could imagine that since they have to say bad things to answer most questions, why not say bad things all the time, i.e. if your wife asks if you like her dress, you have to say you hate it, so why wait to be asked.Really, the film makers did it because it's funny. If you made a movie where people only spoke when they had something nice to say, it would be very boring.
Myridon