While it's true that "de" also is formal you, we hardly ever use it like that anymore. In fact the only situation where i can imagine using it like that is if i had to meet the queen. Whenever you see "de" think "the" or "they".
If you ever talk the queen it would be "De" and not "de". Important distinction. The spelling "De" means you while "de" means them/they
I hear "fisker", well it has been accepted... I wonder if it is difficult for others to hear the difference between "fisken" and "fisker".
Why is it always necessary to have "the" why can it not be "they are eating pasta and fish"
Because this is the exercice for definite articles lol. If it was just pasta and fish, I think it would be "pasta og fisk" and not "pastaen og fisken". I agree though that the sentence you suggested, without any article, sounds way more idiomatic in English.