The discount is from a knowledgeable entity who buys lots of healthcare knowing what they are buying (theoretically preventing fraud) and what competing healthcare providers are selling at, and also negotiating with the collecting purchasing decision of thousands or millions of insureds.
Edit: since I can’t respond to comment below due to hitting too fast posting limit:
Post ACA and pre ACA plans are completely different products due to out of pocket maximum, age rating factor caps, and inability to screen for pre existing conditions.
Hence comparing pre and post ACA health insurance prices is not meaningful.