"The Los Alamos Primer: The First Lectures on How To Build an Atomic Bomb, "Section 2. Energy of Fission Process," page 7:

Somehow the popular notion took hold long ago that Einstein’s theory of relativity, in particular his famous equation E = mc2, plays some essential role in the theory of fission. Albert Einstein had a part in alerting the United States government to the possibility of building an atomic bomb, but his theory of relativity is not required in discussing fission. The theory of fission is what physicists call a nonrelativistic theory, meaning that relativistic effects are too small to affect the dynamics of the fission process significantly."

So basically, our nukes are bigger than Einstein's theory?

Basically because it doesn't make much difference, people are lazy so they just leave it out for less work