why some words don't be in the dictionary ?
I looked at the dictionary(Longman) for this word " germaphobe " and there was not any meaning but in the online dictionary "urbun dictionary " there is its meaning .
now ! is this informal ? slang ?
slang are not in the dictionary ? urban dictionary is a reference for everyday English ?

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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!

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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.

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http://www.medicinenet.com/script/main/art.asp?articlekey=46748

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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?

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because it is not discovered yet