Because Biden isn’t left leaning.
When you stop listening to what democrats say and start looking at what they are actually doing it becomes apparent that the same laws are passing under Biden that were passing under Trump.
There comes a certain point where both parties are so far right that it becomes pointless to distinguish between the two.
At the end of the day the Dems and the GOP are both capitalist bootlickers built on a foundation of racist imperialism and neither of them are going to save us from the problems of the modern day, because they benifit from the systems in place that cause those problems.