Why can bakers’ yeast not use lactose from milk?

Bakers’ yeast, specifically the strain Saccharomyces cerevisiae, cannot utilize lactose because it lacks the necessary metabolic pathways to ferment this specific sugar.1

While bakers' yeast is highly effective at fermenting various sugars such as glucose, fructose, maltose, and sucrose to produce the carbon dioxide required for leavening, it is unable to process lactose, which is the primary sugar found in milk. Because the yeast cannot break down lactose, this sugar remains in the dough throughout the fermentation process.1