That they use millions of computing-dollars to extract this information and store it.
Supposedly spam detection does not need to understand the numbers in your bank emails. Supposedly it needs mostly information about you previous contacts and spammy patterns from the rest of the internet.
The point being that you cannot misuse the information you do not have. Google is very good at thinking of a lot of this kind of quality of life functionality, they do not seem to realize how creepy and disconcerting they can be.