Because its rude to loiter around at the back door??
I've found that hanging a bunch of Basil with the leaves around doorways seem to keep them away. I hang bundles on the back porch when I BBQ and never get bothered. Don't know why, but someone told me about that and it sure seems to work.
They are trying to find a water source and regular mosquito's are temperature sensitive so they are seeking cooler air. I do the same as Rooster and hang basil bunches. You can also get a large bowl and fill it half way with water and drop a mosquito larvae killer tab in the bowl. It draws them in to lay eggs and breaks the life cycle. A handheld propane fogger also works great and keeping them away.
It repels mosquitoes 100%, but it attracts junkie drug-addict cats...