Because Ahmadinejad isn't a person in Western eyes, but rather a symbol of an oppositional power bloc. Netanyahu, GWB & Obama are viewed much the same from the other side, because the views of common citizens are shaped by the propaganda of their leaders.
People are only united in the presence of a perceived "enemy", and demonization of other people's leadership serves that purpose. Carried to the extreme, it leads to war, which the common people on either side don't really want, nor do they generally benefit, either.
It serves rather well to distract the populace from the shortcomings of their own leadership, as well. The louder they scream about the evil somebody, the more likely that they're robbing the populace blind in the background. Witness the Bush Admin.
People are only united in the presence of a perceived "enemy", and demonization of other people's leadership serves that purpose. Carried to the extreme, it leads to war, which the common people on either side don't really want, nor do they generally benefit, either.
It serves rather well to distract the populace from the shortcomings of their own leadership, as well. The louder they scream about the evil somebody, the more likely that they're robbing the populace blind in the background. Witness the Bush Admin.