Average tenure is heavily skewed by massive head count growth. When you add 20,000 employees in one year (Google, 2019) + replace the churn from the population of 100,000 people hopping companies for more compensation, it will drive your average tenure number way down. This is a commonly referenced but completely invalid argument. Very few people are pushed out at the large, stable tech companies.
layoffs happen frequently, the average tenure at any one company is only a few years, and ageism is rampant.
this is hardly a "company for life" role or industry for most.