It's possible that people can actually be former leftists/socialists/communists/etc and have "left the left" so to speak. I've been noticing here that the common response to this will be "they were never really leftists/socialists/etc. anyways" or "they were never really serious to begin with." It's possible that it could be the case, or maybe they actually were on the left, but they just didn't have the stomach for the real shit I guess.
I think we shouldn't be too quick to jump to using the "no true scotsman" fallacy in these cases. For those who don't know, it's basically saying "you weren't REALLY a leftist, cuz otherwise you never would have left." The same argument is used by religious people when someone leaves the faith: "oh they were never a REAL christian, otherwise they wouldn't have become atheist/agnostic."
To give some concrete examples, take Eldridge Cleaver. He actually was a Black Panther, radical, communist, etc. but then later in life became a Reagan conservative. Or Angela Davis. She actually was a radical but now she tells people to vote for Biden.
TL;DR: People can and do change, sometimes for the better, sometimes for the worse.
I disagree, there are many times when I listen to a 'leftist' speak and realize 'Oh man they have absolutely zero idea what they are talking about, this is purely vibes based.' I have spoken to many libertarians who claimed to have been former 'communists' who cannot describe to me the actual differences between marginal value theory and labour theory of value. These are not serious people, nor serious leftists, in general, they simply agree with whatever the popular consensus of who they find themselves around.
That being said, there are people who genuinely leave the left and their reasoning is usually that they believe that conservative stability and American hegemony is better for the working class. I think they are fooling themselves, but if it is articulated well, there is valid reasoning for 'leaving the left', particularly if you are old and just want to cozy up to the establishment. Opportunitism is valid reasoning, even if it is morally objectionable.