Why do many unbelievers want to hold all believers to a criterion they themselves do not want to be measured by?

Why is it many unbelievers want to paint all believers with the same brush because they had negative experiences with a certain religion, denomination, religious parents, pastors, congregation , etc? For example would a non-believing US citizen want to be seen as a boorish, arrogant, dote, and an ”Ugly American” because someone from the US visited there before you and was disrespectful of the people, customs, and beliefs they had? Would you think it fair they believe all Americans to be Neanderthals because a few acted as if they were? If you would not want to be painted a certain way because of a few bad examples, why paint all believers with the same brush because of those who don’t or didn’t get it right?