Part of the problem is non-linearity of delays. If you are delayed 45 minutes in traffic towards the airport, you may miss your flight and have to wait 6+ hours for another. With driving, if you are delayed 45 minutes, then you are just delayed 45 minutes at your final destination.

Because of this, people frequently spend a lot of non-productive time at the airport, because they are afraid of the (small) chance that they can miss their flight entirely.