The US does not have them, and they are not legal. I agree that they could be useful. But in the US you have strictly two options- represent yourself to a court, or let a bar-certified lawyer represent you. Nobody else gets to help in court. Outside of court, legal assistants help lawyers with administrative stuff- doing legal research, organizing paperwork, etc. But the lawyer holds the sole responsibility to the court.
Unless DoNotPay has a strategy to change the law, they are in trouble. It seems that this case was a publicity stunt and not part of a larger strategy.