> Simple - if eating fat was what made you fat, then sheep and cattle wouldn't be fat.......but they are.
...except sheep are not people. As others have said, they are adapted entirely differently from us. We can't pretend that we are sheep or cows. If you do that then you end up DEAD with a gut full of grass like many of the Irish during the great blight.
You probably couldn't eat enough leafy greens in a day to fuel yourself even if you spent all day at it.
We just aren't that good at digesting leafy greens.