Because Google's goals don't necessarily align with your goals (you want to be happy I presume, Google wants to be profitable, among other things of course, but profitably is one of the main drives), and there is no reason to believe they will in the future. It's like allowing someone close to you, know everything about you. When someone have information about you, how you work, what triggers you, what makes you do X or Y, then it becomes a lot easier to manipulate you to suit their needs.
It won't be drastic, it won't be immediate, but it will happen over an extended period of time.
So, if you're fine with a company molding you to that extent, then I suppose being lazy is no big deal.
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