The very simple answer is because dinosaur is a scientific term that covers a very narrow band of fauna, and that bit of popular knowledge is _wrong_... Dependi

The very simple answer is because dinosaur is a scientific term that covers a very narrow band of fauna, and that bit of popular knowledge is _wrong_... Depending on who you speak to. Dinosaurs are "terrestrial reptiles of the orders Saurischia and Ornithischia that existed during the Mesozoic Era, including but not limited to megafauna." There are plenty of other megafauna that the public think of as dinosaurs, that aren't, because they're not part of the clade 'Dinosauria'. So of course anything descended from extinct creatures from roughly the right age are "dinosaurs" to the common person. However! Recent changes in the way we classify species, moving to phylogenetics, says that for a group to be natural, all descendants must be part of said group. Making birds, dinosaurs. Whilst this reclassification (which whilst popular also has popular detractors), wouldn't say people are fish, it would say that both fish and people are Eukaryotes.