Why do installers hang the first sheet of drywall up against the ceiling and not along the floor line?
Great questions,
There are actually a few reasons why this is done. The most common reason is that you want to eliminate gaps. The tighter the sheets the easier it will be to mudd and tape. So when you hang your first sheet you can keep it snug to the ceiling also ensuring it is square to the ceiling. Since the bottom to floor gap will be covered by trim the top is extremely important.
The next reason being that floors tend to be a little uneven. And starting at the floor and keeping a consistent gap so that it lines up is difficult.
The third reason is for the ease of hanging. It is much easier to hang the first sheet snug to the ceiling with out having to worry about if it will needed to be sanded or shaved down. For instances if you needed to an 1/8 of inch off one corner you would have to pull the sheet back down and sand it.
The final reason once the top sheet is in it makes a nice measuring point for marking bottom outlets. Since that is where the sheet will butt up to and floors are far to often uneven. While your ceilings tend to be more square.
I hope this helps.