Why can't you eat or drink before a physical exam?

For a complete physical, there is usually bloodwork done. When you eat something, or drink something besides water, this affects the chemistry of your blood. Therefore, the physician will want to get a pure sample of your blood to examine.

If you eat something with a lot of sugar, for example, your blood will show high sugar concentration and the physician will diagnose accordingly.
If you have eat or drink before the physical exam, your blood pressure will increase so that you can not perform well in physical exam.