The short answer is Life. Just like happiness, money can't buy your way out of poverty. Poverty is a fact of life, regardless of the wealth of a country; the amount of it is not, however. You also have the definition of poverty; those on the high end of the poverty-level would be considered rich in many third-world countries but are barely making it here, given the cost of living in the U.S. As to capitalism being the problem, with one caveat, I would say it is capitalism that keeps the rate so low; I think you will find, as a rule, poverty much higher in non-capitalistic countries.
My caveat is this, the capitalism must be regulated. Unregulated capitalism leads natually to a large poor, although not necessarily poverty, class by its very nature; it also leads to wildly unstable economies; we saw all of this in the U.S. in the 1800s. The poor class comes from the economic advantage a person gets once they become wealthy and the ability to do things to keep themselves in that status. Unregulated capitalism leads to very unequal distribution of wealth where the maxim of the "rich get richer and the poor get poorer" is very true; it is an artifact of unregulated capitalism and we are seeing it today where, in the last 15 years, during the deregulation period of business in America, the gap between the wealthy class and middle/poor class has grown significantly. The growth in deregulation and the growth of the gap is not a coincidence; it wasn't in the 1800s and it isn't now.