Why do cameras have higher megapixels than screen resolutions?

So I have been researching pixels lately. I have noticed that the highest screen resolutions on screens is HD 1080 x 720p (there are some exceptions) which is about 2 MP. Then camera lenses have crazy high MP rates like 20MP and higher, even though the result of the picture comes through back to the 2MP screen. I'm confused? Another ex is the iPhone 7, its screen is also around 2MP, but its camera says its 12 MP.

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