Because, historically capitalism has been the single most efficient machine to lift people out of poverty ?
Another confounding practice is that official US poverty statistics don't include many forms of government assistance. So a statement like '14% of American's live in poverty' is before this assistance. With this representation of data, you could give those 14% $10,000/month in government benefits and they would still be 'living in poverty'.
Here is a nice calculator you can play 'what if' measurement games with to see how it affects the poverty rate: http://uspovertydata.com/povcal/index.php