One theory is that they desperately need to believe in government as parent. When run by other lefties, a benevolent and parental government will shield them from the harsh realities of life by making decisions for them; telling them what to eat, what to drive, what energy to use, what light bulbs to use, how much their toilets can flush, what their children must learn and cannot learn, what they can and cannot say, ad infinitum.
The other theory is that it is a grand conspiracy by the lefties to drive us all to the nut house, leaving the earth to them alone.