Your example is a bit absurd—why would the prediction of human behavior necessarily entail the prediction of behavior in a completely uncontrolled environment, with near-to-zero information on any relevant variables?
Your question is comparable to asking: If physics was so good at predicting the movement of physical bodies, then why can’t it predict earthquakes? If it can’t predict when an earthquake will occur, then it is not successful at predicting the movement of physical bodies.
The point is that we know that humans are incredibly predictable. The reason why humans appear to be unpredictable is simply a product of the vast number of unknown variables acting upon us at any given time. When we remove these variables, and place a person in a controlled experimental environment, the result is highly predictable human behavior. As I pointed out above, we are very good at predicting how people will respond in choice situations, and self-control situations, etc.
The great thing about behaviorism is that not only does it point out that human behavior has causes, but it identifies these causes and quantifies them in simple laws that accurately predict the behavior of individuals. In other words, there’s a reason why behavioral science (underpinned by behaviorism) is considered a natural science.
Why do you say that behaviorism has not been successful at predicting human behavior? Its most popular models of choice behavior consistently account for around 95% of the variance in experimental settings (e.g. the matching law, or the contingency discriminability model). Behaviorist accounts have disproved naive conceptualisations of the “rational agent”, and have developed models of self control which not only accurately predict at what point an individual will choose the smaller-sooner reward over the larger-delayed reward, but they also predicted a previously unnoticed behavioral phenomenon (i.e. preference reversal).
I’m aware of no other area of psychology which has been as successful at predicting human (and animal) behavior as behaviorist theories. The success of behaviorist accounts to not only predict, but also to control, human behavior is one of the features why behaviorism is considered one of the most useful approaches to psychology.