Because (most of) the people of the richest country on earth believe that any form of tax-financed welfare is "socialism", which is evil, and the only reason they're the richest country on earth is diligent avoidance of "socialism".
Wasn't the point of the grand parent's post that, per capita, it isn't higher in the US as compared to the UK?
Homelessness is a moral as well as practical problem. How can more than half a million people have no home in the richest country on earth?