If some engineer optimizes something in the Google search stack that makes it, on average, just 0.01% faster (not 1%, but one-one-hundredth of a percent), then they have paid their salary for the entire year. Almost in perpetuity. No matter what level they are.

Very small gains multiplied out over extremely large amounts of compute over large amounts of time add up big.

And that's why Google can spend so much money on fairly small scoped teams.