As you said, it's because normally it's called an Earth-quake. Which is technically referring to the physical ground/dirt (or sometimes Earth in some cases). It's realistically a little clever way to both differentiate the planet its on and still refer to it as a "quake" in some way. Others also find it funny to name it like that. Idrc, I think they sound a little funky but that's because we call them earthquakes, since we're on earth and it's shaking.

As for why astronauts are referred differently? It's because they're a different culture and sometimes language, not everyone uses all western names/naming scheme. I'll agree I think they should all have one name. It probably came about during the space race and since it was a competition, people wanted their creation to be referenced.