When an airliner is flying at an altitude of 30,000 feet, the temperature of the air outside may be as low as -30 F. However, instead of heaters, air conditioners must be used in aircraft flying that high. Why?

Air conditioners 'condition' the air.

This may be to make it cooler but most have the ability to make it warmer and will maintain the temperature specified by the setting of the thermostat.
So at times they may actually be a heater !
Additionally some can increase or decrease the amount of humidity.
This is important in enclosed spaces especially when every breath of everybody on board an aircraft is increasing the amount of water vapour in the air.

Although outside a plane at 30,000 ft it may be -30F, if reasonable insulation is used (all windows are doubled glazed if you have ever looked) then inside does not fall to such lows over such a relatively short time of exposure.
Also you have to remember that every passenger radiates between 0.5 and 1 kilowatt per hour, so there are plenty of 'HEATERS' on board !!

Actually these air conditioners do no heating...Airliners have pressurized cabins. The air at a height of 30,000 feet is compressed to sea level pressures. This process would raise the temperature of the air to 130 F if air conditioners were not used to extract heat from the air.

The air in an airliner cabin is at higher pressure than the air outside, but it is not at ground air pressure. It is much lower, being the equivalent of the pressure at about 10,000 feet. This is why we feel so stupid on board planes and suffer from blood problems. We are deprived of oxygen.