Cumulative emissions have got to where we are and the US leads.  The US is only behind Canada in per capita emissions.  The US is number two in current emissions. Those are three reasons the rest of the world is looking for US leadership. https://wri.org/blog/2014/11/6-graphs-explain-world%E2%80%99s-top-10-emitters
The Paris agreement (not treaty-you said treaty) is a step in the process which will require many steps. The outcome may be uncertain but is more certain than taking no steps because instant success is not guaranteed.
The likely net economic outcome of not being part the Paris agreement will likely be harmful to our economy. If net economic harm is not a myth, show us the data.