That's your personal bias showing, I'd never be questioning anyone who decided they found a good role where they feel competent and secure about their job and craft, much the contrary as Peter's Principle usually apply very quickly to the careerists.
Senior engineer is usually where a lot of competent folks stop, the role of Staff/Principal starts to become quite different, some people just want to be good at helping the business to build things. A good senior will be the necessary glue between team and the larger org (including the Staff/Principal layer), facilitating a lot of stuff that isn't immediately visible but is definitely felt when you miss it.
On top of that, there's not even close to enough positions to promote every single senior engineer to a higher level in most companies. Not everyone wants to play the career game.
Senior engineer is usually where a lot of competent folks stop, the role of Staff/Principal starts to become quite different, some people just want to be good at helping the business to build things. A good senior will be the necessary glue between team and the larger org (including the Staff/Principal layer), facilitating a lot of stuff that isn't immediately visible but is definitely felt when you miss it.
On top of that, there's not even close to enough positions to promote every single senior engineer to a higher level in most companies. Not everyone wants to play the career game.