Only a few years ago Athens' airport was fondly known to all as Hellinikon and was very conveniently located at the sea shore. Consequently a taxi was a only a ten minute ride and cost only few hundred drachmae. Taxi costs went way up after the airport moved out to Spata where it is now! Count on at least 25 eu for a public taxi.
Originally whomever it is who passes for urban planners in Athens practically made a mantra out the airport being located in Markopoulo. Then just as we got used to it they changed their minds and called it Spata instead. Doesn't matter really as they both border the airport.
Spata is easier to remember but if they wanted to have a name that was easy to remember why did they pick Eleftherious Venizelous as a airport name? 'ATH' to make it more palatable.
If driving in Athens proper or even Piraeus you will notice eventually that they have all these totally out of context signs for the airport in the oddest places. Signs that go 1000 other places before they even come close to the airport. Its confusing. Athens is the worst but Greece can be annoying like that too especially if you are driving and looking for something for the 1st time.
It chronically like this because Greece is stagnating civically.You wont believe it but Greece wont recognize foreign university degrees, like say in urban planning from Harvard, but send thousands abroad every year to study - then they don't listen to them or let them work for the government. The best Greek University is ranked 450th in the world too - still you don't need to go to University just to set up some proper signs do you? You assume we visitors know things we dont!