If roasting meat doesn't generate much juices, why do gravy recipes assume it does?
They may or may not assume a significant amount of pan juices and there are quite a few ways to get more than “just a bit”. Including low-quality meat, bad temperature control and other ways to extract the water from the meat. Unfortunately most are also ways to get dry meat, especially in lean cuts.
Fattier cuts are usually cooked a bit longer and to hotter temperatures in comparison, which will give the collagen time to “melt” and also create more juices as a side effect. The drippings will also contain more fat, than can be used for the roux to thicken the gravy.
There are also lots of recipes that “top up” the pan juices with stock, water or wine (your links both add stock), and small but concentrated amounts of pan juices can bring a lot of flavor to a gravy - just add liquid and boil all bits off the roasting pan, don’t just pour the liquid out of the pan. And finally there are also many ways to “cheat”, from simple box mixes to separately cooked sauces or jus made from bones and scraps.