From the website:
Book now for our direct luxury coaches that take you faster between Oxford, High Wycombe and Gatwick. Departing up to every 60 minutes.
www.theairline.co.uk
Likewise, from National Express:
timetables.nationalexpress.com
Oxford 12:30
Gatwick South 15:15
Gatwick North 15:25
(During the middle of the night it's sightly quicker at 2:12)
In comparison the direct train service is likely to be around 1:45 with limited stops between Reading and Gatwick.
Whist 5 trains a day is on average one every 3 hours and so quite a gap between services, it's not entirely unlikely that someone could be happy to turn up with more time at the airport booked on a train knowing that if the train doesn't turn up they can go and talk to someone and due to the conditions of carriage be likely put into a taxi at no extra charge to them.