There are guesses and there are educated guesses. Good TA practitioners fall in the second category. They acknowledge that markets are unpredictable, for exactly the reason you mention: markets are composed of millions of variables and no one can account for them all. But I strongly believe (as someone who has traded successfully for almost a decade) that rigorous application of back-tested TA methods can achieve success > 50% of the time. Over the long run, that will equate to a sustainable win rate for traders who employ proper risk management.