Cursor is a heck of a lot more than $20/month if you actually want it working for a full work day, every day.
This issue manifests a bit differently in people, but I've definitely worked with people (not only juniors) who only have a few productive hours a month in them. And for what it's worth, some of those people were sufficiently productive in those few hours that it was rational for the company to keep them.
He didn't last long.
You could probably hammer the most expensive cursor API all-day every-day and it would still be a fraction of the cost of a junior dev.