One reason is that Apple specifically blocks their users from selecting a custom Spotlight search provider on iOS. Google is probably willing to pay whatever it takes to lock themselves in as the abusive and immutable default provider.
It's ironic that you framed Google as the abusive provider there.
I consider them both to be exploiting the status-quo. Apple and Google need to be punished for creating systems that exclusively benefit themselves at the direct expense of the user. It's irresponsible, anticompetitive, and the sort of business relationship that necessitates government intervention.