As I posted on here about 2 years ago, that 30 minute flight turned into 5 hours when my plane was diverted to KVA due to fog at SKG which basically ruined my sightseeing plans in Thessaloniki. The train would have brought me there on time - although the point of the trip was to accrue 2 extra A3 segments. I wouldn't have taken a same day return by train just for fun, but someone going for a business meeting might do that.

It's comparable to LHR-MAN flights vs trains. The only time I have flown to MAN I drove 20 mins to LHR and parked at my free parking spot at 0500, had a free breakfast and got to Altrincham by around 0830. Probably same travel time overall by train, except it would have been twice as expensive, and involved a 1 hour packed tube trip to Euston then negotiating Piccadilly station in the morning rush hour. But if I was going to the city centre of Manchester I may have chosen the train.

A Greek resident starting near ATH airport and going somewhere near SKG airport may well find a flight to be the best option, but almost all tourists would choose the train unless the prices are excessive (and the metro to ATH is already exorbitant)