As we seen with the Google Plus bug that exposed user data even if Google would be a nice company even they can have a bug and expose your data, this makes me think that the best solution as a user is minimize the data that is collected about you and as a company to discard any data that you don't need and anonymize the data you have to keep and can be anonymized.
Maybe not intentionally but problem is that often such data can be partially or fully de-anonymized using various statistical analysis techniques.
#1 Reason: no one can monetize that data better than Google
#2 They'd be helping their rivals with such data
#3 Sharing might kill the golden goose (bad press etc)
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Reason #1 sort of invalidates Reason #2. If Google is reasonably confident that they have an unmatchable (in the short term) advantage in interpreting and monetizing your data and can also generate additional revenue by selling it to firms that can't extract the same value from it, what incentive is stopping them? If competitors start to catch up in the areas jn which they have the clear advantage, they can simply shut off the faucet and the revenue generated by selling information jn the here and now helps fund initiatives to keep them ahead in the first place.