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.

Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for humans flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage ... The air is cooled, humidified, and mixed with recirculated air by one or more environmental control systems before it is distributed to the cabin ... Certain aircraft have unusual pressurization needs. For example, the supersonic airliner Concorde had a particularly high pressure differential due to flying at unusually high altitude: up to 60,000 ft (18,288 m) while maintaining a cabin altitude of 6,000 ft (1,829 m). This increased airframe weight and saw the use of smaller cabin windows intended to slow the decompression rate if a depressurization event occurred ...

Pressurization becomes increasingly necessary at altitudes above 10,000 ft (3,048 m) above sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. For private aircraft operating in the US, crew members are required to use oxygen masks if the cabin altitude (a representation of the air pressure, see below) stays above 12,500 ft (3,810 m) for more than 30 minutes, or if the cabin altitude reaches 14,000 ft (4,267 m) at any time. At altitudes above 15,000 ft (4,572 m), passengers are required to be provided oxygen masks as well. On commercial aircraft, the cabin altitude must be maintained at 8,000 ft (2,438 m) or less ... . This cabin altitude is maintained while the aircraft is cruising at its maximum altitude and then reduced gradually during descent until the cabin pressure matches the ambient air pressure at the destination. Keeping the cabin altitude below 8,000 ft (2,438 m) generally prevents significant hypoxia, altitude sickness, decompression sickness, and barotrauma. Federal Aviation Administration (FAA) regulations in the U.S. mandate that under normal operating conditions, the cabin altitude may not exceed this limit at the maximum operating altitude of the aircraft.

https://en.wikipedia.org/wiki/Cabin_pressurization