How do denture wearers maintain a fresh breath considering that they have no teeth to cause bad breath?

Most commercials promote products to indicate that a toothpaste, chewing gum or mouth wash will create a fresh breath. Denture wearers do not have teeth to brush. There should be no more plaque or bad breath from gingivitis or other periodontal problems. Yet, some denture wearers still have bad breath in spite of gargling with mouthwash, brushing tongue and gum, using scented adhesive for the dentures. Are the advertisements just hypes to get consumers money? What really causes bad breath and do the advertised products really help to solve the problem?