Because they've made public statements to this effect, including in the privacy policy. Why trust those statements? Because lying would be a very bad idea. The

Because they've made public statements to this effect, including in the privacy policy. Why trust those statements? Because lying would be a very bad idea. The lies would be revealed very quickly (e.g. via whistleblowers). The outcome would be expensive civil lawsuits, probably billions in fines, and a loss of trust in their $20 billion / year cloud business.