>>14350344It isn't considered a part of the atom's mass for the same reason why 1 kilogram bags of flour are all considered to be 1 kg, even though they might be 1.00005 kg or 0.999995 kg.
Sure, you could use a precise scale and find out the precise weight of each individual bag of flour, but it's not really worth the effort.
Same with electrons. You could take them into account, but the resulting precision wouldn't make much of a difference in calculations. What it would do is increase physicist suicide rate by 300%, because they now have to work with a whole bunch of extra unnecessary digits, every time they do any calculation, for no particularly good reason at all.
