That's because many of these so-called 'leftists' have centred their entire identity around activism and agitation and as such realise that actually achieving their purported goals would rob them of their targets. You can compare them to the pharma industry which is far more interested in finding treatments for conditions than they are in finding cures for those conditions since the latter would cut off the market for the former.
Have you spoken to these people? My social circle has a number of pretty far left folks and while I don’t agree with many of their views, I have not once observed this desire to avoid their own goals that you propose.
I don't think you know many leftists, to be frank.