The reason? Imagine you have a kid, and your neighbor has 2 kids, then you both find out that really, one of their kids is sort of... yours. (-- awkward pause --) That's basically the problem here:
Herpetology is the study of reptiles and amphibians, however unraveling the evolutionary history of these two groups (along with birds) has shown us that birds are actually nested in among reptiles.
Now, to be fair, I think the comic has it wrong: I mean, shouldn't this justify lumping ornithology in a sub-discipline of herpetology?