Why are they cheaper?
If we give the same service as other parking companies at the airport, then why the other companies are cheaper?
The answer is that in reality not all companies provide the same quality service, most collect your car at the airport or at their offices, but then park it at a roofless plot, or what is worse, in the street. They provide outdoor parking without the customer noticing it.
If a parking company doesn´t have to maintain a building, pay security or hire staff, then it reduces the expenses to 0€ and can bargain the prices because small prices still convert into profit...
but also the service you get is at the same level, very poor.