I am very fortunate to have access to the ingredients needed to make my own highly aerated potting mix affordably. I would never use these big pots if I didn’t. I think (this is complete speculation) for those of us with heavy clay soil there is something that bothers me deeply when sticking a bare root tree in this soil. I don’t care what species I’m planting, the success rate will be low for bare root. You have to remember most of my trees are planted in abused farmland that has been drained of every bit of organic matter for at least the last 100 years. With that being said, if I can get a giant root-ball in this soil the plant can breathe. I think plants without a mass of feeder roots simply suffocate in my soil. For me, I don’t believe it has to do with moisture as much as air. My rain cycles are very similar to yours. I believe planting so that the root-ball is at soil level is critical. Mulching the top of the root-ball and surrounding area to suppress weeds and retain moisture are also critical. Basically, a larger root-ball diameter means the tree can exchange more gases, which makes it more resilient.
Once again… complete speculation.