I think there are two reasons one is your rectum has a non return valve and shuts as soon as the fart has been expelled and the second is the methane in the fart has to mix with air to form the right explosive mix for the fart to light .There are other dangers though as the methane can be diffused by the clothing you wear so spreading the burn area and nylon Yfronts can be particularily flammable if you are unfortunate enough to still own a pair
The static charge on those Nylon Y-Fronts caused sparks to fly at fart times and were resposible for unexpected explosions during the 1960s. The US government hushed this up and kept the charred remains in Area 54 using them later for fake moon landings.