DuckDuckGo, Start Page, or Qwant might have been a little more in the spirit of Mozilla and privacy. Then again it takes about half a second to change the default search to your own preference. Google is probably the most approachable for the vast majority of users. It is a sensible move in that regard.
Your right on a privacy front from an absolute perspective. But, it's almost certain that Google paid for this change. A stronger Mozilla (financially speaking) is good for privacy and free software. Also, normal users expect Google. So, having it in the default provides a more familiar experience for them to possibly switch to Firefox.
It'd be nice if DuckDuckGo could afford it, but keep in mind, the privacy aspect of their service is a marketing bent. They still run mostly on AWS. If a government agency wants your DuckDuckGo data, they can just serve a warrant to Amazon instead.