Well this is a question that can be answered many ways. The obvious answer is probably the money, quality of life, equipment, and retirement benefits just to get started.
I have had the good fortune of flying both military, corporate and the airlines. All I can say is that out of all the jobs I have had nothing comes close to the major airline job I have.
Any of you that know any major airline pilots can attest that the life simply can't be beat.
The control over your life is great. Off course it comes at a price, as the present day climate has shown us. Our most junior new hires at the majors are getting furloughed at an alarming rate. Something our industry has not seen to this extent before. But, I am certain that like all good things, this horrible period will be behind us in the comiung years. I will not want to speculate just when, but what I can tell you is that the life is absolutley amazing.
Lets starts with the equipment. It is only the majors that are interested in the newest and most technologically advanced wide body aircraft on the market. Unfortunately the biggest corporate types use a few 757, or the BBJ.
Second, the pay! Look at the recent contracts that have been signed. All pilots love to fly but when you get the top hourly pay based on your aircraft. Life is good, who would not want to make top dollar.
Third, the quality of life is unreal. Yeah you have to serve your time doing the reserve deal, but when you get senior the lines are yours to chose from, and with that goes some great flying. i.e LAX-SYD, ORD-LHR, etc....
Again, talk to your friends or if you have jumpseated on the majors just to see a schedule. You will most likely see some grins and some great flight schedules and stories.
Fourth, the retirement is great! just imagine making great pay after you hit age 60. It is awesome to see what some of the FAE (final avg. earnings) can net you. Take a look at the B funds of many of the majors, and you will see what I mean. This is something not all corparations will do for you. I am talking about pilots making $15-18 million over a 30 year career.
Anyway, the bottom line is that there are a great deal of good flying jobs to have. I know this life is not for everyone, as I have debated it between military, corporate and airline buddies. What you need to do is make your own dream list, and then go and seek the companies that offer what you are looking for.
Please keep in mind I am not knocking any type of flying but simply answering your question as to "Why the majors?" I hope I have given you some perspective on it, and hope that other major airlines pilots will respond as to what they love about this job.