Reach is very "Earth like" for a reason. Fictionally, it's one of the reasons it was chosen for settlement, humans need a specific type of environment to live in and outside of the fiction from a design point of view, it helps the player feel familiar with this strange alien world if players can identify with it.
You are fighting on a UNSC controlled human world and fighting an invading alien force. You are fighting to protect it and your fellow humans on it. We all know that the fight is futile and Reach falls and I think that some of the concept art gives us a glimpse into what the game has in store.
At the beginning, you are right, it looks like Earth. Familiar rolling green hills, blue oceans and rocky mountains. However, Reach is doomed. The Covenant invade and "glass" the planet. By the end of the game there will be little similarity with the environment, the rolling green hills and mountains will be flattened into deathly glass deserts, the calm weather will be transformed into a maelstrom of angry and fiery howling winds. The planet will have transformed from a human paradise into a nightmarish, molten glassy hell.
The Beta maps already offer brief glimpses into this transformation. We can see the beautiful greens in Overgrowth, the calm mountains that nestle Powerhouse and the abandoned human industry found in Boneyard. Take a closer look at the skies, listen to the radio chatter and look far off into the horizons. There's an invading force, the UNSC is outnumbered, explosions light up the sky and a storm is quietly brewing.
We haven't seen the full extent of of Reach's visuals just yet and I believe we won't be seeing it properly until we get our hands on the game. There are surprises in store for us.