All the primes above 2^32 are out, but that accounts for only two point something percent.
The simple explanation is just that lot of ways that numbers can multiply to produce the same product
Like an odd number x times an even number y, x* y produces the same product as x* 2 and y/2
Same for a multiple of 3 c and and a non multiple of 3 d, c * d = c/3 * d*3
But also all of their multiples. I suspect that those account for the vast majority.
The simple explanation is just that lot of ways that numbers can multiply to produce the same product
Like an odd number x times an even number y, x* y produces the same product as x* 2 and y/2
Same for a multiple of 3 c and and a non multiple of 3 d, c * d = c/3 * d*3