Because it doesn't detect bots, it detects people without cookies from google's sites. I've *never* had the captcha detect me as a human.
You have to click a bunch of slow-loading tiles and once you select all the correct ones, it tells you to try again. You'll only need to pass two tests if you're extremely lucky. So along with the fact that it takes at least 30 seconds to complete the captcha, it also sends all your mouse movements and a bunch of other data to google so they can uniquely identify you and collect information about you (hobbies, etc).