The problem with leaving the tip at the end is that the last housekeeper who does the room up for the NEXT person is getting the big huge tip. Now if that's the same person that has done YOUR room the whole stay, that's great, but if it isn't, then it's a real bummer for the person or people who worked for you during the stay.
Really there's no way to ever thank the housekeeper for the job they did on your room, because you never know who it is (unless you are in the room as it is happening). You're leaving a tip out in hopes that the person who gets it WILL do a nice job.