Because Corbyn is more left wing than any Labour leader in living memory, with the possible exception of Michael Foot, but probably not even Foot was this far left.
The far left have been so frustrated for 50 years, if their man were to win, they'd want their pound of flesh; you can bank on it. They'd want him to - as they see it - put right the wrongs inflicted on them over many decades.
This includes restoring their power by bringing back the ability to bring their employers to their knees with strikes any time they like. And doubtless huge pay rises based on them feeling hard done by after austerity and necessary pay restraint. They will demand vastly higher than inflation pay rises, I absolutely promise you.
Because Jeremy is emotionally bonded with these people, he will want to do it as well.
This will further wreck the economy, on top of Jeremy's other economy wrecking measures. He would set this country back 50 years.
Do you remember us being the "sick man of Europe" with strikes every two minutes? I do.