This won't apply to all stews, but if there's something acidic in the liquid (such as tomatoes or vinegar), some items will not soften or break down as quickly.
I know this includes both potatoes, onions, pasta and rice, but I would assume that other starchy vegetables would have similar behavior.
But there can be other factors in play, too. Many people cook potatoes to mash in a fair quantity of boiling liquid, while many stews are simmered. As stews often have liquids other than water, we avoid taking them to a full boil for a few reasons:
- they make circulate inefficiently, leading to scorching on the bottom of the pot.
- if the liquid is viscous, it will create bubbles that pop and make a mess
- it cooks off aromatic compounds ... and anything you smell while cooking is something that's no longer in the finished dish.