Why are relative atomic masses on the periodic tables not whole numbers?

They are averages.

Explanation:

atomic numbers on periodic tables are derived from the average value of all the isotopes of the element. So being averages they are sometimes not integers.

The sum of the number of protons and neutrons in an atom is called the mass number. Atomic mass is never an integer number for several reasons: The atomic mass reported on a periodic table is the weighted average of all the naturally occurring isotopes. Being an average it would be most unlikely to be a whole number.