A couple of main airports are indeed well connected to the public transit network (for example Amsterdam Schiphol has a train station under the airport connected to the main train line and it takes about 15 minutes to get to Amsterdam Centraal Station on a regular commuter train).

However I was talking about the peripheral airports, which are at best on a train line which doesn't have many options (such as London Gatwick) or at far end of a subway line as peripheral airports are the ones that could potentially be faster to use for somebody living outside a main city (which, as I mentioned above, in my own experience was not the case).