It is a simple matter of evolutionary adaptation: de novo lipogenesis is an anti-famine mechanism used to convert low-yield, relatively high process-requirement energy [remembering that glucose -> glycogen conversion is long, takes energy and water, and is comparatively heavy and voluminous] to high-yield, relatively low process-requirement energy.
Quite frankly, this is not merely a matter of cellular biology, but physiology as a whole: even a cursory knowledge of the body's predominant pathways of energy homeostasis [nor/epinephrine, cortisol, T3, and Insulin/Glucagon - of course, this is focusing on the macro, and not the micro of PI3K/Akt dependent pathway of glucose homeostasis, AMPk and so forth] points to the fact that the reversible catalyzing from glucose --> triglycerides [in the reverse, glycerol to glucose, but that is neither here nor there] is highly involved in energy homeostasis as a whole.
Our bodies are the portrait of efficiency, and to suggest we excrete massive portions of nutrients is incorrect. By whatever mechanism - beneficial "muscle-building processes" and adipocyte hypertrophying/proliferating alike - our bodies are built to store energy; such is why we hypertrophy our adipocytes so easily. (Remembering that the diet for the general population circa ~4000 B.C., and onward is characterized by relative abundance from our ancestors; our bodies are built to store energy as a function of high energy requirements + low intake throughout the first 185, 000 years of our history.) In plain fact, de novo lipogenesis is a primary mechanism of this "future storage", so to speak.
Save for certain pathological states, we do not 'excrete' glucose. If we excreted glucose in the magnitude you are speaking about, russ, you would be reaching for a dialysis machine, not a Sandow. (And this is focusing on the cellular level. Such inordinately high BGlu and/or Urine Glucose could be indicative of a massive range of pathological states from HIV to gastrointestinal malignancies such as impermeability or malabsorption to renal failure (kidneys may "recycle" excess glucose)).