Years ago, when I was a single mom, and before I got an education, I did housekeeping as a living. Please know that housekeepers DO earn those tips! It may be different now, but they didn't used to make even minimum wage, and counted on tips just like waitresses.
It is so much harder to clean a room when people stay over than when they have checked out! They still have to clean the sink, clean the tub, clean the toilet, sweep and mop the bathroom floor, make the beds, clean AROUND the guests' belongings, leave fresh towels and soaps, dust everything, empty the trash, check the stationery, and sweep the main floor.
If you want to make sure your housekeeper takes the tip, mark on the envelope "Mousekeeping" and leave it away from your personal items.