Why wouldn’t they care?

I imagine 2 people use roughly twice the bedding, towels, toiletries, etc., on average.

If there's two beds, maybe. If one bed, doesn't matter how many people sleep in it. Maybe twice the towels?

I'll say over the years most places do not respect the "if the towel is not on the floor don't replace it". I'm fine with reusing a towel to dry off twice, but some hotels change them every day, even when their signage is indicating a protocol to prevent that sort of waste.

2 vs 1 doesn’t significantly impact space or cleaning labor, unless you’re staying in a super minimalist itty bitty unit (which I rarely find exists anymore)

Well, they mostly don't. They're often using the same bedding. Most toiletries are squirt containers these days. I don't think toilet paper is that expensive. And another towel or two to wash is probably not a big deal. And to the degree the hotel has a restaurant or bar they probably come out ahead. Your hotel may take a different approach but it's near universal (perhaps outside of resorts) that hotels charge the same for 1 or 2 guests.

Citation needed. In my experience it's entirely the opposite: it's nearly universal that hotels charge more for two guests than one.

You can easily verify this on Booking.com, where the search results show price per room and how it varies based on how many people are in that room and whether they're adults or children.

Price only varies for the breakfast-included option in my experience.

I don't care to do research. I will say when I book on Marriott.com there's never a difference. I don’t use booking.com much though.

Is this a non-U.S. thing perhaps? I’ve never in my life seen a U.S. hotel charge more for double occupancy, outside of special packages that include e.g. meals for each guest.

The other things priced into the "experience" are why I call out cruises separately. With a hotel, I get it, a room is a room. But a solo cruiser is eating half the food, drinking half the drinks, taking up half as many seats at shows...

Maybe that's the problem. Cruises rely on people spending a lot of extra money onboard the ship, or drink packages, nicer dinners, excursions, etc... fewer people doing that, with less social pressure to spend extra, means less money for them and they have to make it up somewhere.

Isn't the cruise capacity still limited by the number of rooms? So saving peanuts on drinks and cheap food probably doesn't make that big of a difference for them.