But then why do you call it "Earth" instead of "Planet"?

Mostly because we'd had the words "Earth" and "Planet" for a helluva long time before we realised that the Earth was a planet.

The sense of "Earth" which came first was the sense of "soil", so our planet is really just called "Soil". It's interesting to think about the process by which our ancestors figured out that "earth" wasn't just the substance underneath their feet, it was also a place.

I often wonder if we'll ever get to call Earth "Terra" like they do TV. ;)

Terra was something the golden age SF writers settled on. It's from the same source (Latin) as the names of the planets and Sol.

Terra is to Earth as Sol is to Sun.

I actually call it Terra. And Sol. And Lua. But that's because I'm Portuguese. :)