Why is doctor who always about Earth?

I love DW To death I love the aliens and the plots!But why is every invasion,every invasion and everything about humans I mean humans are like the prey of every alien.Its like Eath was the only place where to stand on .I don't need a SciFi show to be on Earth because I'm already here!I would like that the doctor helped other monsters from aliens rather than always humans.please answer me!