I had never heard of tipping in hotels until I was an adult. It wasn't something my family ever did. (Midwest US, middle income) I think I've only done it once, when we had some health issues while on vacation and (some of) the hotel staff was really helpful. I put the cash in the envelope and wrote something identifying on it; that was the last day of our stay so I assume someone took it.
I would think an envelope marked "housekeeping" would be the safer choice than just leaving cash out. Although it's not very smart, I could imagine someone emptying their pockets onto the desk at night and just walking out of their hotel room for the day with some cash and change left behind that they intend to come back for. If the housekeeping staff assumed it was a tip and took it, they might get into trouble. Maybe in the OP's case, the hotel has had problems with this in the past, and now the staff are only allowed to take an envelope that says something specific on it, like "housekeeping," and aren't allowed to touch something that says anything else, even "tips."