I've not been on a singles holiday either - but I see where your coming from with this.
The problem is the majority of Holiday companies buy their accommodation in bulk from the hotels - the hotels charge them on a per room basis so the cost is fixed.
The Holiday companies therefore pass that cost on in their prices, so if that room is for 2 persons for instance that cost is divided between the couple paying - so if only one person occupies that room, then that person will be charged the full rate.
However, what is wrong and often happens from what I hear, is that the single person ends up paying the full rate for the room, but then on arrival gets stuck in a smaller single room instead - now that to me is totally unfair.
Another excuse used by the industry is that a couple will spend more in the Hotel for drinks, facilities, tours etc - and these things are often taken into consideration when the hotel sets its room rates.
Yes, as has been pointed out there are singles holiday companies out there that do a great job, but often their prices are still higher than a regular company even though there is no supplement, mainly due to economies of scale and the higher room rates charged to them by the hotels.
At the end of the day the companies are there to make a profit - and they make more money out of squeezing more people into one room.