In most cases I think it comes down to laziness, doesn't seem it would be that difficult to submit a breeders report, they don't have to register them, but they could atleast leave it up to the new owner.
I have a young gelding here now, bought him and his half brother (as weanlings), both out of registered AQHA stock, I was told they were the last foals from this breeder (and they didn't think the mares had settled), so they didn't bother with the paperwork. I got them from a guy that had bought them (and knew the breeders well) in a packaged deal (they had one horse he wanted, but he had to take the group to get it), I saw the other horses they'd got with them, all related to them (included a maternal half sister to one of them) all registered, and do believe I was being told the truth.
I've had one other, got him as an adult from his original breeder, I did get reg. names of both parents, he was a cremello and possibly born before AQHA changed their rules. He still could have been APHA, but the breeder may not have known that, and him being a gelding may not have bothered (I didn't ask why, just assuming).