Antitrust doesn’t care about how you got market dominance. It cares about what you do with it. To use the browser example I used elsewhere: maybe Google’s dominance in search is because it’s indisputably the best. But that doesn’t mean Google can use its dominance over search to get you to use Chrome.
To address your other examples, say health insurance or cellular, that market dominance is absent. Google has 89% of search market share. Verizon has 35% of cellular. Likewise, no health insurance company has more than 15% market share. That’s a completely different market. And I can’t get a discount on cellular because I’m a FiOS customer, nor can I buy car insurance from my health insurance company.
As I said elsewhere, I don’t think an antitrust case against a Google would succeed at this point in time. But lots of things that are common in the web tech space (cross selling products, giving away products for free, etc.), become potential antitrust concerns when you’ve got 90% market share in an industry. For example, giving away Android for free to push Symbian and company out of the market and cement Google search dominance in mobile. You wouldn’t be crazy to try to make a case out of that.