Am I misunderstanding your comment regarding DDG's daily query growth? According to https://duckduckgo.com/traffic they're continuing to grow.

Looking at the whole 2021, both the daily avg. and total monthly queries seem to be flat. Only October and November were positive outliers.

Because it requires low investment and has high margins. Building a device or ecosystem to sell is a lot costlier and riskier than using the data of all the people using your operating system to try and grab part of google’s advertising share.