Even if a user trusts Google, because the data is digital and therefore permanent, there's no guarantee it will remain internal forever, whether that's because of a hack, a rogue employee, police/government pressure, or a change of ownership.
I can’t say I love having Google track me, but I don’t feel any better about someone else doing it either.
Why is Google security better than anyone else? Monopolies often have more resource, but lack motive, because they are a monopoly. Without transparency we have no idea how secure Google's systems are, but we do know Google has been hacked before.