Why must the human body be kept at a pH level of 6.5 to 7.5?
The human body is made up of cells. Inside those cells are enzymes which catalyze the reactions that are needed to keep the cell alive. Enzymes are pretty complex molecules with a lot of folds. They have a primary structure, which is the sequence of amino acids. The secondary structure is either an alpha helix (looks like a curly ribbon on a Christmas present) or a beta pleated sheet (looks like a piece of paper unfolded after you have made a fan out of it). The tertiary structure is the overall 3D shape of the enzyme and the quaternary structure is the shape produced when all the parts come together. All these folds and parts of the structure give each enzyme a particular shape that it needs to carry out its function. If the pH of the human body is either too high or too low, the enzyme denatures (all those folds and structures fall apart) and the enzyme can not function. If the enzymes can not function, then the reactions needed to keep the cell alive can't take place, so the cell dies. If all the cells in a body were to die, so would the body.
The above answer from "nofret" is correct. I wanted to add that the structure of enzymes makes it useful, and changes in pH unravel enzyme structure. These changes in structure are referred to as "denaturing" the protein. You can denature a protein (enzyme) by changing the pH or raising the temperature. This is why we can cook foods with both temperature and acids (pickling).
Think of a screwdriver. Its shape makes it useful for putting in screws, where a saw's structure makes it useful for cutting. Raising the body's pH is equivalent to breaking the tip off of your screwdriver. Now, it cannot perform its function.
As to why the human body has particular pH levels, the enzymes have evolved in our body to function at these optimal levels. By keeping body pH fairly constant, these enzymes are able to be specific in their function.