Because that way they avoid any competition.
Thing is, businesses like Google’s ads are not linear. If you can track 90% of people 90% of the time, your ads are much much more valuable to advertisers than a company that only tracks 70% of the people 90% of the time. So it makes sense to create a moat by literally shitting money on everyone around you.
Think about the opposite: if Apple would switch to DDG by default, most people would leave it at that. And that would mean, a significant chunk of the US search traffic is gone. Europe and the rest of the world are not that apple-heavy, but Apple users are rich power users (on average), these are extremely valuable.
I think you’re missing the point here. You’re claiming Google only pays Mozilla to have a competitor, yet they also pay apple even more money for the same thing in an area they’re just competing.
The point is that there is competition in the default browser search space, it’s just that Google pays more than anyone else.
If Google stopped paying Mozilla tomorrow, someone else would pay them for the same default search engine spot. It might not be as much, but it would still be a significant amount.
A few years ago it was Yahoo that footed the bill.