The atomic weight is the average mass of all of an element's isotopes (isotopes are atoms of the same element with the same number of protons but a different number of neutrons, and protons + neutrons = atomic mass.
Each isotope of an element has a whole number of protons and neutrons, but the atomic weight is the average of all the isotopes found, and it is affected by the observed ratios of the different isotopes. If the number was a whole number, then no known isotopes have been found (rare other than some man-made elements).
Short answer: The reason the atomic mass is not a whole number is because it's the weighted average mass of the isotopes of that specific element.
Because the atomic mass is a calculated average of all the Isotopes of an Element with their % of which they occur also calculated in.
The atomic mass of an element is the average mass of all its isotopes, weighted by their abundance. For example, the mass of Carbon is given as 12.011, since Carbon 12 is by far the most abundant, but Carbon 14 also exists, shifting the average mass slightly above 12.
