In days past when players spent time between the baseline, and the net the half volley was a legitimate stroke which was drilled, taught and practiced because it was often used in matches.
The way tennis is taught now the only reason a player would have to hit a half volley is because something went wrong, which is why it is seen as more as an improvised thing now.
I see it with kids all the time these days.
Jolly: "Did anyone every teach you how to hit a half volley?"
Kid: *Blank stare*
Where players of old would hit a half volley and come in, younger players just stay back and hit a groundstroke.
We stopped checking for monsters under our beds when we realized they were inside us.