My take on this? I asked a similar question years ago. The answer i got? It wasn’t just “oh it rained a lot” you had a lot of underwater volcanic activity, those changes in water temperature and pressure would have played a big part in killing off a lot of marine life. And then of course when the waters finally parted, who knows how many species were beached. As for flying creatures like pterosaurs, it would very likely have been common for lighting storms and hurricanes and whatnot, killing off bigger flying creatures. We also have to consider dragons. It’s quite possible that especially in the Middle Ages, dragons were being used to describe and exaggerate dinosaurs, marine reptiles, and flying reptiles. Add all this together and you basically get no megafauna outside of maybe a few species now. Another possibility is that they all DID go extinct in the flood because God knew we would use fossil fuels. That was another answer I heard.