A significant fraction of Mozilla's funding comes from google paying to make google the default. I am not sure what the dollar value of this agreement was, but it was a sizable fraction of a billion dollars a year last I checked.
Because Google pays them to do it and essentially to a large extent sponsors Firefox's development. Would you rather have Firefox die due to lack of funding?
I expected Firefox to ship with DDG as the default but alas it did not to my surprise. Why is Google still the default in Firefox?