As a junior, you are more desperate for work to get experience, have less liabilities and would accept anything anyway, so the salary won't make much difference.
As a senior, your time worth more and you should not waste it on companies you are out of their league, you are less likely to accept a low salary as your liabilites need to be covered.
Yes, you can negotiate but your prespective assumes most companies would give you what you want eventually, which isn't true.
As a junior, you are more desperate for work to get experience, have less liabilities and would accept anything anyway, so the salary won't make much difference.
As a senior, your time worth more and you should not waste it on companies you are out of their league, you are less likely to accept a low salary as your liabilites need to be covered.
Yes, you can negotiate but your prespective assumes most companies would give you what you want eventually, which isn't true.