Medical Question about Colonoscopy?

I heard that by the time the average person reaches 40yrs old they have 7lb of undigested meat in their intestines. Although obese people have more. It was reported Elvis had 42lb, and Marlon Brando had 63lb. My question is when you have a colonoscopy they drink a gallon of Drano, Not really, but it is some kind of cleaning stuff. You sh*t and then kind of pea out your , for 3 hours. Then when they do the medical test, they also flush you out. So when you have this done, are we not starting fresh, as clean as a baby, and we start clean? If not why?