Because Congress didn't need to find out any facts. They just wanted to put on a show to sway public opinion.

Remember when they questioned Toyota over unintended acceleration? That made Toyota apologize and generally made the public believe that Toyota was at fault.

Now, consider that at the time Congress questioned Toyota they were the largest shareholder in Toyota's biggest competitor (GM) and, when NASA examined the evidence, they found no issues in Toyota's products.

If you want to talk about securities fraud, that Congressional hearing should be at the top of the list.