I think this reasoning is flawed. Joining Google does not mean you get to solve interesting problems because you will most likely contribute incrementally to a vast body of work. You want to build the next-gen database? You have to come up with something way better than BigTable and Spanner. You want to build a queue service? You've got to come up with something way better than Google's PubSub, which optimizes itself all the way down to the level of GBP protocols. You want to build a machine learning framework? Have you checked out TensorFlow and JAX in particular their ecosystem? You want to build a file system? Are you sure you can overcome all the organizational inertia that Google has built on Colossus over the years? You want to build a 10X more productive framework for data processing? Are you sure you can beat DataFlow or Flume4J-ecosystem in Google?

The point is, Google is a mature company. Mortals like most of us don't get to break ground in a technically advanced but mature company like Google. Instead, we find fast growing new problems to solve, to hone our skill, and to get to scale.

P.S., I personally know a number of prominent professors used to work on the Borg projects just to optimize for a few percent of gains. It's deep and interesting work, but nonetheless hard for mortals like me to get much out of.

That is, it's a more sure bet to work for a baby Google than to work for a middle-aged Google.