If I had to guess it's because they see the term leftist as another meaningless term like progressive, which I kinda agree with. I think refusing to be labeled a leftist is dumb but I definitely hate when people say "leftism"
Idk if you guys realize this but basically everyone who makes their money as a public figure, especially if they've been one for decades, is against cancel culture. It's because they're scared that they said something 20 years ago on a recording that will get them cancelled and they will have to work normal jobs.