Because the same amount of ethanol is going to be less concentrated in a larger volume, since cells are basically just globs of watery liquid (cytoplasm) inside a bubble of mostly lipids (a cell membrane, although bacteria do also have cell walls, unlike human cells) the same solution of alcohol ends up somewhat more diluted in a larger volume of liquid than it does in a smaller one (you also have to account for the mechanics of osmosis on a larger surface area and other factors like that, but the end result is pretty much the same either way). Even then it’s not a huge difference, and any solution of ethanol concentrated enough to kill a given bacterium would have a decent chance of killing an isolated human cell also, but this hardly matters with the comparatively small concentration of ethanol in mouthwash, especially since the epithelial cells that line your mouth are constantly dying off and sloughing away in large numbers anyway.