Why are flies gathering on my covered patio in the same spot?

Hi Hometalk Peeps...I have a problem on our back covered patio. Every year in the same spot we get tons of flies ....We put up one of those fly traps before that traps them in a plastic bag filled with water....Problem is the same trap that kills them actually attracts them....I just want to know why they are always there in the first place. Do they like covered patios?
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