People coin new words every day, especially online. I've not seen "germaphobe" but it's an example of a made-up word. The meaning is clear but the word may never become popular. If I published an article online with the word "workaphobe" (a person who is afraid of work), I could not really expect Oxford to publish it any time soon!

We Americans tend to make up words at the drop of a hat, so many of our expressions may not appear in the official dictionaries of the literary world.

By way of correcting you headline: Why are some words not in the dictionary? Or. Why can't I find some words in the dictionary?

because it is not discovered yet