How are they not easier to tell apart? They are because they wore costumes so if you knew what costumes they wore then you know who it is. Plus alot of them had their own little trademark looks.
But new bands don't have costumes, they could be wearing different street clothes every night, so how do you know who is who?
Honestly - tell me its not easier to tell Motley Crue, Poison, Warrant, Van Halen & Guns N Roses apart than it is to tell um The Vines, The Strokes, The White Stripes, Jet, etc apart.