“Leftism” is a response to the failures of liberalism to uphold it’s own values. A bunch of people wentt, “hey this freedom and rights for all stuff sounds pretty rad but it’s not really happening under the system that claims to be making it happen. How would a truly free society actually look, and what do we need to do to get there?” So yeah, it’s based in liberalism inasmuch as leftists are calling liberals hypocrites that failed to uphold their own values and goals