Actually they don't, carbs are broken down into glucose, fructose and galactose. Glucose, generally, requires insulin to be absorbed by the muscles, liver and fat cells. However Fructose doesn't, the cells then convert it into whatever form necessary, the liver usually converts it into glucose for storage. Yet muscles are capable of using fructose directly.

Glucose can directly be turned into ADP or ATP, or it is converted into Glucose-1-phosphate, which then can either be turned back into glucose or follow the path to be turned into Fructose-6-Phosphate (fructose incidentally directly turns into this). F6P is then converted into triose phosphate, a change that cannot be reversed, and can be directly converted into fat. Fats, when needed, are turned back into triose phosphate and can basically be directly converted into ATP.

Essentially fructose needs less processing than glucose to be readily stored. The aim of the human body is to increase its fat reserves for winter when food becomes scarce. Glucose is readily available and readily usable by the body, but all this depends directly on insulin to get it into cells to be useful. The notable cell that doesn't require insulin are brain cells, however these readily process fructose too.

So it's actually possible to completely remove glucose from the body, in the event it was ever needed then fructose can be back-stepped into glucose-1-phosphate. However, the body can more than adequately produce energy directly from fats, so it would seem extremely rare that the body would need glucose.