Why people who, for genetic reasons, do not produce the lactase enzyme in their bodies

Why people who, for genetic reasons, do not produce the lactase enzyme in their bodies, cannot metabolize milk sugar lactose, but dairy products (yogurt) can?

Lactose is a good substrate for gut bacteria. If a person does not have lactase, bacteria begin to digest milk sugar on their own. Fermentation occurs: bloating, nausea, flatulence.

Dairy products are pre-fermented: microorganisms are added to them that convert milk sugar into lactic acid (which is why yogurt is called a fermented milk product). Lactic acid is well absorbed by the body and does not cause fermentation.