Therein lies the problem.
On the two Boeing types I fly, there are either 3 temperature zones (including the flight deck) or four. The temperature selection range is from 18c to 30c marked in 2 degree increments, although it can be set to any temperature in that range. Obviously the mid-point temperature is 24c.
The actual temperature set will depend on many variables. If the aircraft has been sitting for some time in very warm or very cold ambient conditions then the cabin will be often be set at one or other extremes in order to achieve a rapid warm up or cool down as may be required. Much as you might do in your car in similar circumstances.
Similarly, if it is daytime in a warm country where people are used to air conditioned buildings and transport, then a cooler environment may be set to reflect the expectations. During the flight the temperature may well be raised to reflect the change from day to night, or peoples greater sensitivity to cooler temperatures as a result of inactivity.
Truth is, if you leave the temperature at a mean average, you can bet that someones request for an increase will becomes someone elses request for a decrease without any change occuring at all.
With three temperature zone controllers, you can guarantee three people being happy. The rest will either be happy or unhappy to varying degrees depending on their own circumstances. Perhaps we should fit those placebo type thermostats you often find in hotel rooms, where you can move the dial to convince yourself that something is happening, but nothing actually is.
Even if you could get the temperature just right for the majority, there will be variations where it is warmer near the inlet ducting. Cooler near the windows, unless the sun is streaming in. Cooler near the door frames. Warmer near the galleys.
Best to take a personal mini fan and a blanket and modify your own micro climate.
My belief is that the cabin crew set the zonal temperatures based on their own comfort levels - bear in mind they are normally seated next to the doors, which will be cooler than elsewhere.
Cabin crew usually don't set them (although they may on certain aircraft types, and I wish they did!) In fact the cabin crew are only seated near the doors for a short period during take off and landing. Normally they are very active and working in galleys. Generally they request cooler temperatures for these reasons.