Because when it comes to hacked pokemon, it's very possible to, say, push the stats a LITTLE past the maximums, and it would as first glance look legit if the advantage is small enough. And nobody is willing to check the stats of each and every pokemon (that's 12 per battle to check), to see if all the stats conform to the accepted maximums. (bieng 31 IV + 252 EVs + Beneficial Nature). And since there are 6 stats to check per pokemon, that amounts to 72 checks per battle, which is most certainly a pain in the $$%#. That is a reason I'd see, anyways. I don't actually know for sure.
But that scenario seems quite likely to me. No hacks, so nobody has to worry about stat checking. Just because you trust every single human being doesn't mean there isn't just ONE person willing to play a little beneath the belt to come out on top.
This is what happens when you think alot guys...is this a good thing?