But there is: the Council of Bishops, as guided by the General Conference. UMC churches do not even get to choose their own pastors: they are appointed by each annual conference's bishop.
The Council of Bishops is not the governing body of United Methodism.
An individual bishop can do whatever and the Council of Bishops can complain, and that’s about it.
The General Conference meets once every four years (same years as US Presidential elections) and has become increasingly dominated by UM's from Africa who are, unsurprisingly, much more conservative, Biblically grounded, and theologically orthodox than their North American counterparts.
This has put the liberals from North America into a tailspin. They don't want to play the "white supremacy" card like some parts of the Anglican communion, so their only apparent alternative is to leave the UMC.
"Apparent" because there always is the preferred option of repentance.