The four terrestrial planets are so called because they ?

Well, you need no look further than the word "terrestrial" If you notice the beginning of the word you notice that it consists mostly of the word "terra" Terra by definition is just land. Due to the solid land of these 4 planets, they're called terrestrial planets, the other 4 aren't made of land but of gas which is why they aren't classified as terrestrial planets.

Are similar to earth.

100 pounds is 100 pounds no matter the object or item being weighed