Think of it this way: there are billions of types of online interactions where detection is either impossible (an image or link post) or extremely difficult (general conversation where sometimes even humans don't sound like humans due to slang/education/etc).
Not only that, you'd end up with a "tug of war" where the existence of such detectors would power the improvement of the bots, which would require the improvement of the detector (which is always more difficult).
And the other option is an anonymous token that defines you as a human user. Which is simpler and cheaper to implement?