isn't miner collusion rational?
No. Your premise is wrong. There is no guarantee a solution exists in a range. If a miner exhausts a nonce range he changes other components of the block header. Mining is a Poisson process with each hash attempt independent of any others.

Hence, there is nothing to gain with this particular kind of collusion. Of course, pooled miners enjoy less variance than solo miners (which is legitimate), and there are other methods of attack.