Why is booger not in the dictionary?

"Booger" is actually in the dictionary. It's listed in both Merriam-Webster and Oxford English Dictionary.

You might be thinking of the fact that "booger" is a slang term and is considered informal. Dictionaries often prioritize more formal language and may not include every single slang word. However, "booger" is widely recognized and used, which is why it's been added to reputable dictionaries.