Why is the mass of electrons generally disregarded when calculating the overall mass of an atom?

The mass of electrons is generally disregarded because it is extremely small, approximately 1/1836th the mass of a proton or neutron, making its contribution to the atom's total mass negligible.
The vast majority of an atom's mass comes from its protons and neutrons. While electrons do have mass, it's so tiny in comparison that it doesn't significantly impact the overall atomic mass, especially when calculating the mass number.