Why is the molecular formula for a disaccharide not double that of a monosaccharide?

The reason the molecular formula for a disaccharide is not simply double that of a monosaccharide is because when the covalent bond is formed between the carbons of each sugar, an H of one is removed along with an OH from the other. This resulting loss of an H + OH forms H2O (water), therefore the reaction combining the two sugars covalently is called a "dehydration synthesis."
For example, in combining 2 glucose (C6H12O6) molecules, instead of it being 2 × C6H12O6 = C12H24O12, the dehydration synthesis removes 1 H2O to make room for the C-C covalent bond between the two sugars' carbons. Therefore it is 2 H's and 1 O less than that doubling, or: C12H22O11