(not battle) Why do dragonball feats vary so much?

Frieza, Buu, Piccolo and most of the Saiyans are seen as planet-level warriors. I'm not disputing this fact, but would like to ask how come the only planet seen vapourised in the manga was namek, especially with manyn characters quadrupling their power since the namek saga. How come they are seen as only mountain-level in most fights?

This may work better in general discussion

@deadlake: Well, I guess they are holding back when shooting in the general direction of the planet. Kuririn was shocked Vegeta was using Final Flash on earth. Doesn't earth have plot armor in most fiction, anyways, its not like any other known planet busters do more than devastate a landscape on earth.
I mean once that was gone Kid Buu almost destroyed it with one of his regular Ki attacks and then actually destroyed it with a Ki attack a few dozen times bigger that he did not have to charge up.

This. And to add to it. Is because the writer himself do not always remember his previous work. So he going to do a lot of things that are the same and have been done before. The Spirit Bomb should be useless by now because it take too long to charge it up and leave them weak and open for an attack.

Because if they destroyed the earth in every fight there would be no place for them to fight. The whole purpose of the Z fighters was to protect earth and most of the villains are more interested in fighting the characters(mostly Goku) than in destroying the planet.