Every branch of the Armed forces has different reponsiblities, tactics and uses. If every branch was called the same thing it would just get to confusing. You have to understand how big our military is, Its enormous!
Why would it be? The Army would take over the Corps before the other way around. The Marines are meant to hit a target and move on. You need a bigger Army to attack a target and hold onto the land it has taken.
Every branch has its own specific job. Marines are about sending a few soldiers with advanced training to attack a location. Then they send in the Army to occupy that area will lots of semi well trained soldiers.