The left’s wrong base assumption is in the nature of man.
They claim the inherent goodness of man where the reason people go bad is outside influences instead of internal moral problems. Fix society, and you’ll fix man.
Problem is, you have to fix the man, individually on a person by person basis, before society is fixed. Nothing else will work because external control, by law and thereby government, cannot produce the internal control in a person to be a better person and not do wrong.
Only the Jesus they reject can do that.
That takes a lot of work, first to identify the problem and then to come up with solutions. People who lean left believe that one-size-fits-all can fix everything. They don't want to have to analyze individual's problems; they don't even want to have to analyze each of society's problems separately. They just want a simplistic "solution" that will magically fix everything and create utopia.