Sad side of the US- Being Homeless

When I was walking on the route of the lady bird lake, I came across with a tree full of names. It was looking very colorful from far, but when I came closer, I recognize that the tree with names is prepared for a memory of passed away homeless of Austin City.
It was sad because these people lost their life on street. Maybe because of cold, maybe famine, maybe disease. It is a shame for the US to left people live in this conditions.
I don't know why being homeless is common in here and I'ld like to know. Some say they choose to live on streets, others say they some of them are mentally ill.
Long story short, this is not the American Dream, this is American nightmare, this is madness.
Pay less money on weapons and army. First, sustain the citizens humanistic lifestyle, at least sustain each person's main needs like shelter, food, and education.