Not all of whom are working on AI and machine learning. Most are working on network and server software development.
Also, Google hires many developers today. That may or may not be the case in 10–20 years.
Google, like all large corporations, hires a range of talent levels, from entry-level to senior. It’s safe to say that many of their hires are not exceptional; they work on mundane programming tasks, which every corporation has.
Even at Google, mundane jobs will be automated away. Even they want to cut costs wherever possible. And labour is very, very expensive.