Because "prediction" here doesn't mean "magical prediction of the future", it means "spotting and extrapolating patterns", and (in so far as the Efficient Market Hypothesis is true) there are no exploitable patterns to spot in stock-market data.

(Presumably the EMH is only approximately true, but it's probably close enough that Google can't make billions that way without a considerable risk of losing billions instead, and without a lot of effort that they might do better to put into making billions by more conventional means.)