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.
Turnover is definitely not uniform and a 12 month architecture lifespan is not, in my experience, either normal or healthy.