"The term [meat] is sometimes used in a more restrictive sense to mean the flesh of mammalian species (pigs, cattle, sheep, goats, etc.) raised and prepared for human consumption, to the exclusion of fish, other seafood, insects, poultry, or other animals."

https://en.wikipedia.org/wiki/Meat#Etymology

I think it's just a general apathy in the general population about knowing the difference. "Meat" is pig, cow, horse, rabbit, elephant or whatever, "fish" is fish meat. At a restaurant you could ask if a salad has any meat, they say no, and when it arrives, it has tiny pieces of bacon because "oh but that's almost nothing, certainly not \"meat\"".

Christians are allowed to eat all food. Only Christ is what saves. (Mark 7:19)

But the fish / meat / etc is a tradition thing, so it comes from the culture surrounding the Christian, and probably more relating to Jewish history more than anything

It's some good ol' catholic rules bending.

During lent, you weren't supposed to eat meat as part of your fast. However, not eating meat is... not as enjoyable as eating meat, so they basically declared that fish doesn't count as meat so they could eat it without breaking the fast.

For similar reasons, they also declared beavers to be fish later on.