It is because most liberals do not believe in God.
In a world without evil, in their view, God is not necessary. So that is their utopia. They try to explain away evil as your past determining who you will be in the future and the choices you will make.
In their view, if you fix your past so there are no "root causes" for evil, then...you won't be evil.
Whittaker Chambers said it best in his foreword to his seminal book Witness, in his letter to his children:
"It is not new. It is, in fact, man's second oldest faith. Its promise was whispered in the first days of The Creation under the Tree of the Knowledge of Good and Evil: "ye shall be as gods." It is the great alternative faith of mankind. Like all great faiths, its force derives from a simple vision. Other ages have had great visions. They have always been different versions of the same vision: the vision of God and man's relationship to God. The Communist vision is the vision of Man without God."
It is a brilliant statement of the vision of Communists and Liberals.
It's a form of projection, casting off uncomfortable thoughts. Being caught up in a cycle of vanity, envy, and hate, leftists are evil incarnate. If evil can be imagined away the heavy load of personal responsibility is off their shoulders.