The reducer means that everything attached to the receiver is going to be 2" and not 2.5" and so it will not be as strong. Why would anyone expect a 2" ball mount couple as strong as a 2.5" one?
The 2" steel ball mounts are rated for 7500 lbs. while the 2.5" ones are rated for 18,000 to 20,000 lbs. which is not that surprising. Why make a ball mount that that is twice as strong as the receiver in which it will be used? The Class IV receivers are rated for load of 10,000 lbs. maximum so no reason to have a ball mount rated for twice that amount.
My truck's factory receiver is rated for up to 13,000 lbs. so its capacity would be greatly reduced with a 2" ball mount that is rated for only 7500 lbs.