Not just airlines. All pilots are required to have sufficient fuel for a flight plus extra fuel sufficient to divert to another landing place. Once in the air,

Not just airlines. All pilots are required to have sufficient fuel for a flight plus extra fuel sufficient to divert to another landing place. Once in the air, you can't just pull over and park until the weather clears, or the fog lifts, or the sun comes up again. Like you can in a car. And during the landing itself, you need to have enough fuel to be able to 'go around' for a second or third (or more) attempt on landing in poor conditions. Landing is the most difficult part of aircraft flying, you need all the extra help you can get. Silly to spoil everything by being forced to land because of the lack of just a few litres of fuel.