The Latin Rite has modified abstinence rules over time. In the early Church, the practice was more like that of the Eastern Catholics and Orthodox. No meat, including fish, no eggs, cheese or oil. Fasting is much more rigorous. Since eggs, fish, (nowadays even meat broth) are permitted in the Latin Rite, it’s become almost a custom in some places to have seafood on Fridays during Lent. In fact some live according to the letter, and not the spirit of the Law, eating shrimp, lobster, and other expensive seafood. I myself enjoy fish-sticks, they are cheap, easy to cook, not as good as salmon or sword fish, which I would prefer, they remind me of my childhood when that’s what we got growing up. Not a major mortification, but it’s what I do.
I do sometimes go for meatless including fish, eggs, and dairy. But not everyone feels they have had a meal without some sort of animal protein. I’m Hispanic, and I know many Hispanics don’t eat a meal without some sort of animal protein if they can avoid it. It could be cultural, or a sign that one is successful enough to afford some sort of meat. But among the very poor meat is a treat once a week or less even outside of Lent.