It's because the carrier bearings are so large a clam shell type puller will not work (at least I have not come across one large enough to cup the bearings). For this reason I had to buy new bearings when I put in my ARB even though my truck had relatively low mileage at the time.
I was able to get my ring gear on without heating it. Most of the time a dead blow hammer will seat the ring gear over the pilot just fine. In some cases (other applications) I've had to heat the ring gear and freeze the carrier because the tolerance was that close. A toaster oven works great, DO NOT use an open flame to heat the ring gear!