IBOC on AM is, sadly, mostly used to create intentional interference to competing, adjacent-channel, out-of-market stations. I've talked to more than a couple engineers who would just as soon shut it off, but management fights to keep it on, for the reason stated. Getting AM ratings is an all-out war nowdays. Perhaps if they offered something different and interesting to capture listeners, that would help, but that requires far more talent and money than typical penny-ante corporate types are willing to pony up.
FM... most are simply using HD subchannels as studio-transmitter links. A handful actively program them, but they all know market penetration is nil, and outside of major markets, will never garner any kind of meaningful ratings. The few that do, are basically broadcasting former popular programming from stations that were flipped to other formats.
A few companies simply cannot leave something that works well intact, whether it tanks the ratings or not. There are an incredible amount if idiots in radio management, working for companies who cannot afford to spend anything because all the money is flowing to stockholders, massive debt and operating costs. The fact so few HD radios are in the wild is compounded by the fact most people cannot be bothered to figure out how to use them if they *do* own one.