Stupidity.
Or more specifically: the fact that in democracies the stupid, who are very large in number, have a disproportionately huge influence on how the world shapes up, solely because they far outnumber the few people who actually can understand what needs to be done to ensure that the right persons, who are capable of ruling in the best interests of most, are elected. The stupid are, sadly, not confined to any one area of the planet in particular and their influence has reached pandemic proportions. The ideal system of voting would, of course, give greater "vote weighting" to those who are less likely to be stupid (doctors, degree holders etc). But alas, the stupid masses would naturally revolt in disgust at their disenfranchisement (despite their having difficulty even spelling the aforementioned word!!) and thus the present system is doomed to continue forever more.
Oh dear... (btw, the ever present realisation of the above is partly why those of us who are smart aren't exactly blissful for most of the time.)