Whichever way you choose to look at it, Earth is a place where 10 billion people live. That alone should be enough for ANY Shepard give a crap. Failing that, it's the place where Anderson is and if you don't care about that, them you *must* have been indoctrinated after all.
Small aside, but whatever Shepard's origin, she/he at some point must have visited Earth (Rio de Janeiro, specifically) since that's where the Interplanetary Combatives Training program is based and were the N1 course is held. The off-world training doesn't start until the N2 - N6 courses. And if that's not enough; the Normandy starts it's journey in Earth orbit right at the beginning of Mass Effect 1, so Shep most likely embarked as XO on the ground.