The added amenities are a large part of it. Since you're profile shows you're from the US, let's use Southwest.
A large portion of their costs savings is in their route structure. They don't rely on the hub and spoke system that AA
and others use. The aircraft fly into say, ORD
, and then back out. Connections have to leave at similiar times to allow passengers to get from one flight to another. The result is banks of flights where everything is on the ground, then a lot of downtime while they fly to the outstations. Because of this the legacy carriers have to have a ton of gates, and enough personel to handle those flights. Southwest on the other hand does a lot of point to point and through flights.
This gives a lot of bennefits. Southwest can get away with fewer gates, and need fewer people while having just as many flights. Their employees are more productive throughout their shifts because they are working more flights rather than sitting in between banks. Southwest has some of the highest pay rates in the industry but they save a ton of money because of the productivity of their employees.
Another big point of savings is the fleet. Many airlines have realized the need to reduce the fleet types. Specialization has some benneifts but overall it's better to limit the number of types within a fleet. By doing so you need to stock fewer parts, you don't need as many ground service items like towbars, and you also save money in training because each type is a little different for ground crews (door operation, towbars, etc) and when you mistake the procedure for opening on the door of an airbus with that of a boeing things can break and that costs money too.