* Google sees the writing on the wall: people are becoming more privacy conscious, but more importantly governments in many countries are making motions to suggest this sort of tracking could be regulated in the future. They need to stay ahead of the curve.
* The value of tracking people in this way may be less than everybody seems to think - not all data about a person's activities is equally monetizable. Just because you read an article about Julius Caesar and then looked at some clickbait article about Britney Spears doesn't mean an advertiser can sell you a chariot.
* Google in making the first move can create an advantage for itself in the market
* Much of Google's revenue comes from their own properties -- in the case of search, they are already well positioned to show you relevant ads because you have shown intent. No need for cookies there.
* Google has a dominant position with Chrome and the article suggests they want to do fingerprinting of a person's browsing behavior and put people into different cohorts without sending the raw data to Google. This is similar to how Apple does machine learning on device. If Google has exclusive access to these cohorts, they have a strong incentive to move to a cookie-less model because it puts other ad networks at a disadvantage.