They are probably used to tourists just bending over and taking it raw, rather than complaining and pointing out it is a rip-off (which it is).
Don't give me the high rents excuse, the airport was built with taxpayer's money and the AOT is 70% government owned.
A good comparison would be with the airport in KL, Malaysia, where prices of food vendors are much in line with those in the city centre.
Food prices at Thai airports are a rip-off, end of.