To get back to the OP, I think that the reason that “pure” mathematics tends to avoid putting the commas in numbers is to avoid any complications with writing things as ordered pairs and other tuples. For example, you can write one of the solutions to the Pythagorean theorem as
3704955[SUP]2[/SUP] + 4939940[SUP]2[/SUP] = 6174925[SUP]2[/SUP].
And as you can see, there is little difference between this form and
3,704,955[SUP]2[/SUP] + 4,939,940[SUP]2[/SUP] = 6,174,925[SUP]2[/SUP].
But, such solutions are most commonly offered as Pythagorean triples which look like this:
(3704955, 4939940, 6174925).
Using commas in the numbers would give rise to
(3,704,955, 4,939,940, 6,174,925),
which is much harder to read, and some people are not so good at putting the spaces after the commas, like (3, 4) gets written (3,4). So you get a giant visual mess.
Remember, also, that the higher you get in math, the less likely you are to use actual specific numbers larger than about 8. And when you do use them, you are most often generating a list of solutions to equations, of which there are often many. So, a compact way of writing them down, with tuples, is generally preferred. All such math books that I have in my possession write large numbers in this way, with no spaces or commas or anything else between thousands. That is more of a custom in the sciences and finance.