Not all airports are "so cold all the time". Some do not have air conditioning and may be even hotter than high external temperatures (due to press of people, and equipment) and some are practically outside anyway (eg Cabinda before the new terminal was built).
20/21 °C is roughly the temperature inside the cabin of an aircraft, during daytime, in the air. Most people seem to find that comfortable and dress accordingly. There is no reason why they should suddenly feel uncomfortable at that temperature just because about to board, in transit or having just arrived.
Inability of fire to spread at 20/21 °C as a 'reason' is just 'journalism'. The fire risk is probably greater (if negligibly so) the colder the temperature inside than out, since the compressors have to work harder.
Al Hamad Airport Doha is merely an example of how cold 20/21 °C may feel when coming from 40 °C. I very much doubt you saw any of the people that spend a long time in the airport (eg those who work there, but also some transit passengers) shivering. I do though recall that from about 20/21 °C to ~40 °C felt like entering a furnace and the transition had much more impact than a steady 46 °C and an outside temperature well below 20/21 °C.