why is fish not considered meat if its the tissue of a being once alive?

Fish is absolutely considered meat by anyone rational. It is flesh cut from the bone of an animal.

The reasons the Catholic Church say fish isn't meat are historical, not particularly rational, and are inconsistent with the biology. You know that they considered seals and whales to be fish as well, right? And bats to be birds?

They don't have to be rational and they don't have to be correct, but when discussing science we do have to be rational. Some Church claiming fish is not meat simply makes that Church incorrect. Easy peasy.

Fish isn't considered meat because English and Latin are slightly different languages. For hundreds of years Catholics were not allowed to eat meat on Friday. But the language of the church is Latin, and what Catholics were not allowed to eat is 'carne' which is the flesh of creatures from the land or the sky. So fish was fine.

Fish is of course meat. It is not vegetable for sure. Fish is considered more healthy than other kinds of meat, such as beef, which is called red meat. Therefore people who advocate a healthy diet often ask people to eat fish instead of red meat. Somehow, for some people, they synonymized the term meat with the term red meat. Therefore they do not include fish within the category of meat.

Strictly speaking we could call it meat. Fish, just like beef, lamb, pork, is the muscle of an animal. It has always been classed as different. I don't know how the difference arose but there is no actual difference.

vegetables and fruit are also "tissue from a being once alive"

The distinction between meat and fish is not a scientific one but instead based more on belief and custom - and with belief and custom there is no "why"

It was brought about by the Roman Catholic Church to allow the bishops and above to eat meat on Friday.

Because we consider fish "white meat," as most Meat is Red. But that's what I've always been told so I wouldn't know how else to answer it.

how is it not meat, vegans don't eat fish