It's because pricing of cruises (and many other package vacations, like tours) are intended for double occupancy, but advertised at a per person rate which makes it seem like a smaller number, and singles have to pay a premium for using that space. Its just the way this sort of travel has always been priced.

Cruises appear cheaper for two people because the entire cruise industry pricing model is built on double occupancy. Most cabins are designed with two beds or a convertible king, and the "per person" price you see advertised is calculated based on the assumption that two people will be sharing that room and splitting the "real estate" cost. When two people occupy a cabin, the cruise line maximizes its revenue potential because they now have two passengers spending money on high-margin "extras" like drink packages, shore excursions, spa treatments, and specialty dining. If a single traveler wants to book that same room, they are often hit with a "Single Supplement" fee, which can be as high as 200% of the per-person rate, essentially making them pay for the "missing" person's fare. In 2026, while more ships are adding "Solo Studios" to cater to individual travelers, the bulk of the fleet remains focused on the two-person unit, as the "onboard spend" from two guests is much more profitable than the revenue generated from one, even if the room is full.