Doing one doesn't prevent them from doing the other. It gives them more data about you to sell, while limiting the dataset to gullible rich people who think that you can capitalism yourself out of the problems of capitalism (the prime advertising target demographic).

But what is the incentive?

Money?

You are the one claiming that Kagi is somehow special, trustworthy, and immune to the same influences that drags down everyone else.

Like all the companies that realized that quietly selling highly targetable user data is more profitable than their actual services? See all the "DNA analysis" companies selling data left and right