Remember that the architecture competency in FAANG exists in the context of a 2-3 year average tenure. And this elides a tendency to change teams internally even more frequently than that. It is unlikely that a senior engineer rated "Exceeds" at design and architecture in Silicon Valley has ever learned from or been evaluated on the consequences of their own decisions at more than 4 years out. Median probably one year.
An average tenure hides a good number of people with longer tenure than that. I'm personally a counterexample to your point, for instance, and I know quite a number of others on other teams at my company (one of the FAANGs).
Turnover is definitely not uniform and a 12 month architecture lifespan is not, in my experience, either normal or healthy.
My experience is that architectures usually last 2-3 employee tenures, the point is the original architect usually doesn't see the outcome of their work.
