Historical argument: Model R/C aircraft have been around for longer (including a video downlink and limited autonomous flight like return-to-home) than the term drone has been applied to them, even if it existed before as you said. Since the capabilities of these R/C aircraft didn't change, it is logical that their name doesn't change either.
Usefulness of the definition: Aircraft which are the size of a van and shooting rockets at brown people in the middle east are also called drone. Since capabilities, dangers and use case differ so radically from a 600g aicraft crashing into trees because it's fun, it is logical that they have a different name.
Political argument: Because drone is associated with the aforementioned military aircraft, it makes it easier to call for more regulation of hobbyists by playing on the public's fears.