Google has a heck-load of money. They could pay a disreputable aggregate company to deliver a load of tonne-bags of gravel to my front garden, blocking my car in, and generally destroying the landscaping. I trust them not to do this, and the reason this is true is because I haven't taken 100%-effective steps that would prevent them from doing so, and therefore they are capable of it. This example may seem a little ridiculous (there are no 100%-effective steps I could take, and I think they're trustworthy on this particular score), but it is a technically correct example of how everyone trusts Google.
Putin or Kim Jong Il or Brad Pitt could also pay to get gravel delivered to your address. You have not taken any specific 100% effective steps to prevent them, any more than you’ve done with Google, right?
You basically get a (more) privacy focus OS from a luxury brand or you get your OS from an advertising company.
1. Buy a standard android but spend ages trying to de-google it.
2. Buy an iphone, but then I just have to trust another trillion $ company, and pay much more too.
3. Live my life without a phone.
I choose 1 because that seems the least worse option right now.