So its not like restaurants where they can legally be paid under min if tips make it up but rather that housekeeping tends to be immigrants (often illegal) and thus underpaid. Most hotels will allow you to opt out of room servicing but when you leave they still have to “turnover” the room. Those neat sheets and clean towels all get thrown in the wash. The carpet still gets vacuumed, etc.

All that said I don’t really follow through on this all the time especially as I don’t carry cash as much nowadays.