Google pays device makers enormous amounts of money to direct search requests to them. 20% of Apple's profits come directly from Google.
Estimates from the two companies suggest that it would cost Apple $20 billion in development costs and $6 billion per year in operating expenses to replace Google Search with its own platform, and when Google is paying you $20 billion a year not to do that the decision is pretty simple.
The only problem is that if you are deemed to have a monopoly, which doesn't necessarily mean an absolute monopoly, this is illegal.
Exactly what will happen is still anyone's guess, but Google has few friends on either side of the political aisle. Deemed insufficiently on board with the progressive side of the kosher sandwich, the company has burned every imaginable bridge on the conservative side, many of them twice.
@Aether@truckface this was already public knowledge fwiw but you're right there's a bunch of interesting stuff in there (I'm about 1/3 of the way through it.) One of the things I found interesting was that Bing is, in user tests perceived about as good as Google. But they tried entering into agreements with various vendors and companies told them to fuck off because they were a distant second place in user share. If you're not number one you're trash. I don't know how antitrust legislation would "fix" this, it really is a natural monopoly.