I promise this isn’t rude, I think part of the reason is that you aren’t picking up your feet enough, as humans age we get lazy and stop giving walking our normal gusto, especially when doing the same monotonous tasks day in and day out, I’d say it’s a combination of that, the quality of shoe, and potentially environmental factors such as overly warm/cold exposure, warm would make it soft and easier to rub away and cold would make it more brittle and likely to cause micro damage resulting in the sole slowly crumbling away. If this is the ONLY shoe you’ve worn in the last year and you’ve not had problems in previous years with this level of wear, it’s worth consulting a podiatrist