Because Google knows it's search engine has degraded in quality over the years, and though Google Search might be better than alternatives that Apple could swit

Because Google knows it's search engine has degraded in quality over the years, and though Google Search might be better than alternatives that Apple could switch to, last time they had this fight (where Apple Maps became default) Google permanently lost 60% of their userbase on the platform to a lower quality competitor, a folly which they don't want to repeat. Maps is not profitable for Google or Apple, which is why they are willing to play games with it. Their experiences with Maps definitely inform future decisions though!