Hi Nayi, there are two important reasons why you don't get sugar by reacting carbon with water. First, the reaction is endothermic (DHo = +918.25 kJ), making it unfavorable although not impossible energy-wise, because we are going "uphill" in energy from reactants to product.
918.23 kJ of heat energy + 12 C (graphite) + 11 H2O (l) --> C12H22O11 (s)
But more important is the extraordinarily high specificity of this reaction. Potentially, a mixture of many thousands (at least!) of compounds containing carbon, hydrogen, and oxygen could result in a random fashion from mixing carbon and water and heating it up under various conditions of pressure and temperature. For it to give only a single product, and such a highly complex one in structure as sucrose, is, from a statistical point of view, extraordinarily unlikely. Kind of like the monkey striking the keys on a typewriter at random and producing a perfect copy of the works of Shakespeare - no way!
A term related to this statistical concept is the entropy change for the reaction. Reactions that go to a state of greater entropy, or greater randomness or "disorder", are more favorable. This reaction does the opposite - going from a more random mixture of water molecules and carbon to a highly specific, "ordered" sucrose product. The standard entropy change, DSo, is -409.36 J/K for this reaction (a favorable entropy change would be a large positive number instead).
So, we can't make sugar from carbon and water in the lab, but remember that plants can make sugars and other complex molecules in a stepwise process through the miracle of photosynthesis, starting with carbon dioxide and water. Many highly specialized reactions are involved which use special catalysts called enzymes to make them possible - reactions impossible to do in a test tube! There are zillions of photosynthesis sites on the Internet; this one at http://www.flyingturtle.org/photosyn/chloroplast.html gives you some idea of how complicated the "machinery" is that accomplishes the extraordinary synthesis of carbohydrates and sugars in the leaves of plants. Hope this helps! (Not a "stupid" question, believe me!) 
Steve
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