Because the drop/puddle is trying to keep its shape. I think that's what the current top comment is saying about the contact line being fixed. On rough surfaces

Because the drop/puddle is trying to keep its shape. I think that's what the current top comment is saying about the contact line being fixed. On rough surfaces the edge can't just retreat as it evaporates, and if I understand correctly it also wants to keep the rounded shape at the edge due to surface tension, so water gets pulled in from the rest of the puddle to fill it out.