It's not just the time to get through the airport, it's allowing for traffic on the way to the airport. Maybe not so much of a problem for some airports but for others you need to legislate for significant traffic delays. I used to live in North Manchester and the journey to Manchester Airport took anything from 30 minutes to 1hr 30 minutes on any given day/time of day. So, you had to allow the 1hr 30 mins just in case. Funnily enough, when we had to drive down to London it was easier to gauge time as the longer distance evened out the traffic jams.