The reason is imo probably more along the lines of that the model can't see the actual numbers you're giving it.
When you provide GPT3 with the number "10011111110100011100011011110010", it actually sees [3064, 26259, 1157, 8784, 18005, 1157, 18005, 8784, 1157, 3064, 940] [see 0].
The number isn't even split up into even parts. Even if it knew the Peano axioms, it wouldn't have a chance because it can't 'see' the actual numbers that you're feeding it. Hence it can only estimate these operations based on combination pairs it has seen in training data, and it seems pretty obvious why this can't be a robust basis for doing arithmetic.