Google built their empire on contextual advertising, but it's not the only kind of advertising they do these days.
Also, Google's revenue isn't necessarily derived from what ads provide the highest ROI; it's derived from what ads people want to buy. Regardless of what anyone at Google thinks about contextual vs. behavioral ads, it's in Google's interest to go nuts with behavioral ads simply so that they aren't letting all the other adtech companies keep all the behavioral ad spend uncontested.
Also, Google's revenue isn't necessarily derived from what ads provide the highest ROI; it's derived from what ads people want to buy. Regardless of what anyone at Google thinks about contextual vs. behavioral ads, it's in Google's interest to go nuts with behavioral ads simply so that they aren't letting all the other adtech companies keep all the behavioral ad spend uncontested.