Tipping housekeeping at a hotel is "the way it is done"
The "way it's done" in your circles or your area or whatever. However, that's simply not true everywhere. It just isn't, despite what you want to think. It's never been my experience in all my years of travel - alone or with other people.
I do tip in many places. I tip at restaurants and pretty much all of the other places mentioned. However, this is not one of them. I give my money away to many people in tips, but I kinda resent the fact that people are pretty much making those of us who are not accustomed to tipping hotel staff feel like we're subhuman or violating some kind of "social contract" that really isn't there across the board.
If you want to tip someone, it's great. And, seriously, more power to ya. I'd love to be able to tip every person I come across "just because" they did something. However, at some point, there is tipping bleed that starts to be called a "social contract" that's not really true. Waiters in restaurants are not the same as hotel staff. They're not paid the same way (possibly with exceptions, however, I'm speaking in a general sense). So the tipping is not a required part of their pay.
Like I said earlier, if there is a significant or unusual type of mess, of course I'd do something extra. That would be instinctive. However, the basic hotel visit does not require the same kind of clean up as others have mentioned with vomiting or other things of that nature.
I think that there is, to some degree, a "holier than thou" attitude here (even if it's trying to be subtle) of those who are tipping in this situation. I would bet, though, that there are places that I tip that you don't see a need.