Airports are almost never in the city center.

Train stations are in the city center, plus in Europe they’re always connected to the rest of the public transportation network, which generally means the subway.

If you live within the urban area getting to the train station is invariably a shorter trip, if you live outside it depends if you’re lucky enough to live nearest the side of the city were the airport is or not.

My own experience when living in London before Brexit is that for example to go to Paris, even when I lived just outside the Greater London area and on the side of the city with an airport (London Stansead), door-to-door going to Paris by flying didn’t end up being any faster than taking the Eurostar train from St. Pancreas and even back then you had to go through passport control for the train because the UK was never in the Schengen Area so taking the train didn’t shave of that time.

Part of the problem is that peripheral airports aren’t anywhere as well connected to the public transportation network as city center train stations are, so often the best way to get there is by car, by which point you’re either doing an Uber or Taxi to it (which if you’re outside the city is actually a bit of an expense) or you take your own car but then you have to park it which unless you’re doing a daytrip or such means longer term parking areas, which are further away from the actual boarding gates, and all that shit adds up. And then on the other side you have the exact same problems but in reverse order, so you “pay” the overheads of having to go through an airport twice each way when you fly.