Why shouldn't it be? lol

By decimals, I assume you mean the decimal fractions such as 35.5 for Cl. There are two reasons for the decimal fractions. First, atomic mass is the sum of protons and neutrons; the latter are slightly more massive than protons and therefore produce slight fractional increments; second, atomic mass is the average of the sample and many elements such as lead are mixtures of several isotopes; averaging these results a fractional value.

These values shift slightly as one or another atom is taken as the standard. Originally O was taken as 16 and other values calculated on that basis; then C was set at 12 and all the other values revised. And then ... well you get the idea; With these factors set in play, fractional values for most of the elements are inevitable.

That is the E=mc2 problem. When the nuclear fusion and fission acts, they take or release energy. That causes the weight of each proton or neutron differ.

Martin seems to have the right idea.

I always thought it's because there are several isotopes of elements, and the atomic mass is an average of all the different isotopes, depending on the percentage amount of them there is found naturally in the ground.

But accurate isotopic masses are also decimals.