I’d say that we are past the 50% mark.
The real reason people quit their jobs is because the managers charged with ensuring their employees' success don't care enough to meet their needs as valued employees and human beings.
And this is consistent with leading research by Gallup. In one study of 7,272 U.S. adults, Gallup found that 50 percent of employees left their job "to get away from their manager to improve their overall life at some point in their career."
Gallup CEO Jim Clifton summarized this unfortunate phenomenon in a succinct sentence when he said this in the State of the American Workplace report:
"The single biggest decision you make in your job--bigger than all the rest--is who you name manager.
When you name the wrong person manager, nothing fixes that bad decision. Not compensation, not benefits--nothing."