I believe it is primarily a result of ignorance and self-denial.
At least in the USA, a lot of people went their entire lives unaware that there is any difference between liberals and leftists, and used the labels interchangeably for themselves.
Then, at some point along the way, they encountered actual leftists online and were told differently, so rather then confront that they've been accepting and perpetuating a falsehood about themselves all this time, they find some "legit" leftist label they can cosplay as instead. Engaging deeper with their new adopted costume label is something that hasn't happened yet for them.



