Or is it that the victims pest warning system is currently winning the biological arms race, in which case how are mosquitoes able to successfully reproduce? Or is it that mosquitoes have evolved such that their spawning numbers offset the difficulty they have biting?
Biology is hard.
My understanding is that when they bite you, they also inject a bit of anticoagulant to prevent clotting as they suck blood out. The foreign material creates an allergic reaction that itches.
I don’t think there would be much of an evolutionary advantage to irritating your victim until they start a pesticidal war against you and your kind.
They just seemed to have the correct attitude to do it anyway.