John, it's not just levodopa that the COMT enzyme acts on, it's all the catecholamines as well.

From Wikipedia (see link):
"Catechol-O-methyltransferase (COMT) is one of several enzymes that degrade catecholamines (such as dopamine, epinephrine, and norepinephrine), catecholestrogens, and various drugs and substances having a catechol structure."

Catechol-O-methyl transferase - Wikipedia

In regards to your main point, if you had a drug which contained just levodopa and entacapone, you wouldn't be able to stop the DDC enzyme from converting a lot of the levodopa into dopamine, before the levodopa made it into the brain. The dopamine produced in this way would be of no benefit to the PwP, since dopamine cannot pass through the blood-brain barrier.