The real reason for which is of course that … Dinosaurs are Too Big to Fail. :)
Discovery of a couple large aquatic lizards preceded the invention of the dinosaurs by a few decades. Most notably among these, we find mosasaurus, ichthyosaurus, and plesiosaurus, which looks dubious and which Cuvier thought was a hoax.
I read the German Wiki article on mosasaurus, and even though the creature looks straightforward by comparison, and I find no reason to doubt its existence in a configuration roughly similar to what's presented today, the science isn't settled at all in the matter, and there's considerable questioning and shuffling about of the various specimens and the corresponding taxonomy, which is because the evidence is scarce and sketchy, by the very nature of the fossil record.
Far greater and deeper questions arise when we turn to the dinosaurs. Fundamental things like their posture (wrt to their tail) have been "corrected" in the past decades, and it looks like a couple decades from now they'll actually have feathers to be more in line with how they've been inserted into the evolutionary graph as the ancestors of birds. Other things such as the unlikely (and unviable) anatomy of many of the most famous dinosaurs don't look like they can ever be fixed.
With all the bona fide and even sympathy for such phantastic paleontological scenarios, I simply do not understand how this situation can be a base for any kind of "certainty". Curiosity and fascination, okay, I get it – but certainty?