Going from Jr to Sr in an organization is "easy/ier." Making the same progress as a freelancer/contractor is more difficult.
NB: This is if you were defining junior/senior based on pay. Hard to tell from your comment if you were.
I agree the whole situation is a bit ridiculous. But when in majority of companies salaries for developers are limited to something like 60-70k.
This has been slowly changing and there are more companies willing to pay up to 90k but even then as contractor you can make equivalent of 120k or more even after still taking 25 days off per year.