why do employers take looking at facebook profiles of a potential employee so seriously?

[Kenneth]

You say that a facebook profile doesn't "actually portray" someone. That is absolutely incorrect and wrong. Nothing portrays you better than a profile of yourself that you created and added content to. What you choose to put online is a portrayal of yourself, whether you want to believe that or not. Anybody can put together a fancy resume and act like they'd be a good employee when being considered, but what is on your FB wall and profile is a much better representation of the real you then the version you want employers to see.

Employers look into a lot of things, they do not base hiring decisions solely on FB profiles, but a profile that shows that you may be a problem employee absolutely can and should be a dealbreaker.

This question sounds like you got turned down a job over your profile. Instead of complaining you should use this as a lesson, take down the picture, clean up your profile, increase your privacy settings and move forward in your job search. This is the real world, anything and everything that you put out in public can come back to bite you, so be careful with how you choose to represent yourself online.