The second factor is far more important than the first. There are any number of engineers who can simply go read up on the fundamentals as they are needed, or

The second factor is far more important than the first. There are any number of engineers who can simply go read up on the fundamentals as they are needed, or who understand them well enough to work with them but lack a mathematical proof-level command of the material. Google just doesn't need to hire them because they're paying much higher than market rate.