They are ALL fractional!
only C-12 isotope's mass is not fractional, because it is defined as 12 u exactly. But also in the case of carbon the atomic mass is averaged among the natural isotopes, which include 1,1% C-13. Consequently, the average relative mass of element carbon on Earth crust is 12.011 u.
So, one reason is that atomic mass is averaged among all the isotopes of each element.
You should observe that the most similar atom to C-12 which is probably C-13, has relative mass 13.00335 u, that is fractional (see reasons 2 - 3).
Even those elements that are made of a single isotope have a fractional atomic mass, because the mass of protons and neutrons are different from the atomic mass unit, and they are even different from each other (neutrons are slightly heavier than protons)
Furthermore, the mass of an atom is not equal to the sum of the masses of isolated neutrons, protons and electrons due to the different binding nuclear energy (the most stable nuclei have less energy and less mass). In other words the mass a single proton or neutron is different in different atoms, because these particles interact differently in different nuclei. Differences in energy correspond to little differences in mass ("mass defect") due to E = mc².