In my experience, I've seen plenty of promotions from within (and experienced my fair share as well). They just take a while to happen. But then, I've been at multiple companies for over 5 years. That's probably about how long it takes to watch someone grow from an individual contributor into a leader, including leadership skills as well as building reputational and political capital across the organization.
Your anecdata doesn't align with actual studies that indicate frequent job changes as the optimal strategy for maximising income.
Your loyalty isn't worth anything to your employer in the current market. Those times ended several decades ago.
Here's some counter anecdata: after graduating, I doubled my salary and seniority twice in 3 years with two job hops. If I stayed at my first employer, I'd be lucky to be making a third of what I am now and none of the career prospects that have become available.
I'm in my mid 30s and have a seniority level that lifers at my current employer hope to achieve sometime in their mid 40s if ever.
As someone who is not and has not worked for a FAANG or in silicon valley, I would say that the parent comment's observations are still (generally) correct.
I appreciate your perspective. I am not claiming that they don't happen (though in your experience it may be more common). I am claiming that when they do happen, the primary, initiating cause is a need on the part of the business/management to fill a role.
A person may be operating at a level that would qualify them for the new role for quite some time, but unless and until management have a need for someone in that role, the promotion won't happen.
That's why (in my experience) promotions happen more frequently when finding a new job: the fastest way to make sure the employee's readiness for a promotion lines up with business need is to find a business that already needs someone for that role.
Patience is a virtue.