Traditionally job hopping has been looked upon as unfavourable but nowadays it is seen as far more acceptable.
Job hopping can open up a number of opportunities beyond just salary negotiations; it creates new openings to additional development and responsibilities.
The reality is that internal promotions are typically very controversial. For every one promotion, several other employees have their noses put out of joint.
It can be far less controversial to helicopter-in a new external hire. Existing staff have no knowledge of their shortcomings and the hiring manager can create the new joiner as something of a superstar. It makes sense to appoint a superstar, right?