At a prior company, in order to be promoted from associate to senior staff, you had to demonstrate and be evaluated performing at senior level while associate for a year. In order to get hired as a senior, you just talked with a hiring manager and an HR rep, checked off some boxes based on a few blurbs about what you did at prior jobs, and boom, you were a senior. Same thing with getting a raise. To get a raise to 150K (just as a randomly selected round number), it was an enormous bureaucratic process going up 5 levels of management that would take months with lots of aggravation - and unlikely to really pan out. To get hired at 150K for the same role, it was a couple phone calls with an HR rep and hiring manager - about 15 minutes in total.
One struggle as a result is that the levels themselves slowly shift upward; it's harder and harder to get promoted as time passes. Other companies tend to have level compression, which Amazon's approach attempted to address. Now Amazon has a problem in that level 6, accepted as the typical terminal/career level, has become larger and larger as it stretches upward.
The external interviewing process into that role must demonstrate that the candidate has been performing at a level that is higher than average for that role. The promotion process takes 1-2 orders of magnitude more time for a known internal quantity than it does for an external unknown quantity.
There is a marked focus on improving the recruitment process but little on improving the promotion and retention process.