Because then SEOs would write bots to keep clicking that their site is relevant to everything.

But you can get SEOs to fake metrics, too.

because the underlying assumption is that what they'll tell you is the truth, and that's not necessarily the case. Think of a Firefox plugin in, AdNauseam style, that always says NO.

But there's nothing stopping the same people from gaming existing logic that tracks user behavior except security through obscurity. But you also get dirty data via tracking where it's indistinguishable from backend if user found what they want or just gave up on trying for example.