Because it is a clear sign of world leadership that you simply boycott human rights conferences that include examinations of your own record.
I mean, Russia wouldn't go to something like that if people were being mean to them. So why should the United States? Amirite?
In an Unprecedented No-Show, the U.S. Pulls Out of Planned Human Rights Hearing
Trump's holdy handy friend Mrs Theresa May has a similar plan, as she wants to get rid of the human rights we have here. Obviously, it is not in her best interests to have subjects who can not legally be tortured and killed by the state.
It's at least a bit of honesty on the US part. Since for decades we just talk the talk and rarely walk the walk when it comes to civil or human rights, especially when it comes to our meddling in the affairs of other countries.
It's been a joke and a game for decades, and usually only a sledgehammer you use to punish governments we do not like while we shield those we do like or sweep our own issues under the rug.