Because the UN has exactly one purpose: Serve as a forum for nations to communicate and prevent world war.
That's it.
You can read the US' reasoning for it here: https://geneva.usmission.gov/2017/03/24/u-s-explanation-of-vote-on-the-right-to-food/
The US also voted no because the belief that States are responsible for implementing human rights provisions themselves, not requiring other countries to pick up the tab. The US already provides more than half of global food aid. This resolution would have amounted to "all of your food belongs to everyone" which is bullshit.
