"Why does a dragon, or any large monster, need to have a deeper meaning beyond a potentially hostile encounter as a gate between party and treasure?"
I don't think there's anything wrong with acknowledging that every monster in D&D has a history in the real world, often tied to traditions and criticisms of those traditions.
Dragons especially are corporations and aristocracy; they hoard wealth, using impenetrable defenses and power both social and physical to take whatever they want from the world around them.
But, then, I am the person who sort of believes that "meaning-making" is more or less the core reason people exist at all, so. Grain of salt, I guess.