@smokey I'm sure it is a combination of variables and also business models. Facebook, for instance, likely has the talent and budget to do anything at all - so if anyone could do it, it'd be them. But they make money based on your seeing the Feed. So that is likely a factor. Google, too, has the chops. So you'd think YouTube would remember what video you were on, but they also might prefer you to see their home feed.
I'm guessing it is technically possible, even if difficult, but that the motivation isn't there for most apps.