The high turnover rates are documented as a desirable outcome. They are baked into the process. No less than the New York Times called out no less than Amazon for said-same.
A year or so is the average stint everywhere, because there are a lot of options and for most of them the only real requirement is knowing the tools, so changing jobs is easy. If you're Google or Amazon you want to keep the people whose primary qualification is knowing information retrieval, or machine learning, or..., you don't care about people whose primary qualification is being able to make python sing and dance because the guys who can make python sing and dance can make python sing and dance anywhere, and are liable to leave because they want to play with a new toy, or have an offer near their favorite pizza place, or...