less than 200 years ago, during the mid 1800s the term dinosauria was created and casually during that time fossils of the alleged extinct creatures started being found everywhere. The academia and people interested in pushing this imaginary creature started finding them, how convenient.
the alleged existence of this creature supports the theory of evolution, which was also created during the mid 1800s.

humans have been around for sometime and people never found anything like that. And there should be millions and millions of fossils everywhere if they were real.
because of movies, books, toys and the "education" system, people nowadays see a bird and they imagine a mythical creature, a dinosaur.