I am going to build a new house on slab foundation in Georgia. My question is can I run my main water supply line in a 2" schedule 40 to an interior wall up to the attic and drop down my water supply lines to each fixture or do they have to all run to the exact locations under the concrete?
I know that the drain lines do but don't understand why the water lines do.

If you are not in an area where the pipes could freeze, then there is no reason why you can't run them in the attic, the advantage is that they would be accessible, under the slab they become very hard to repair if necessary. In Calif. They do it both ways.