Some sugars are not fermentable by yeasts (but are by gut bacteria), also there are proteins that add body, mouthfeel and flavor in the final product.
Yep, trub is the sludge.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk