That's easy. Because if Apple were to disallow the use of the google search engine on their phones and web browser they'd find themselves slapped with an anti-trust monopolistic practices suit faster than you can say "Google Lobbyist".

I mean, if they wanted to, they could try using the whole, "customer privacy" excuse in open court. But like the famous meme says:

"Oohhh that's a bold strategy Cotton. Let's see how it works out for 'em?"

I wouldn't be too sanguine about their chances of surviving such an openly hostile action to a clear competitor.

Apple does the next best thing, it gives you the option not to use Google. But it doesn't make that choice for you.