This is incorrect. Far from unheard of, the civil justice system in the US is, in fact, frequently used as a secondary route of recourse by private persons, regardless of the existence or outcome of a criminal prosecution. The legal test used to determine liability (in civil justice) is different from the legal test used to determine culpability (in criminal justice), and therefore, you can also have mixed outcomes. Furthermore, civil law in the US focuses on "making the plaintiff whole", vs criminal law which focuses on sanctioning the defendent. As a result, civil cases might be the only way in which a victim of a crime could get any form of compensation from the perpetrator.
One extremely high-profile case in the US was, of course, that of OJ Simpson [1], who was found not guilty of murder in a criminal case, but liable in a civil one, for the exact same action.
[1] https://en.wikipedia.org/wiki/Murder_trial_of_O._J._Simpson