Why are Doorman or Concierge jobs so difficult to get into?

I've often heard that people who have jobs as a Doorman or Concierge usually get that job in their 20s, or 30s, and just stay with it until they are well into their 60's, 70s, or past whatever the current retirement age is supposed to be for anyone employed anywhere. For anyone to stay with a job that long, obviously it must pay quite well and have very good benefits. Why is it that Doorman or Concierge jobs are so difficult to get into, or apply for, especially in New York City? I even tried asking one of Donald Trump's doorman at Trump Tower how to get a job as a Doorman, but instead of a direct answer, he mostly gave me a run-around story saying, "Oh you have to go to this place, and then go to that place, and then see these people". The main thing people also say a lot is that you have to already know people who do that type of work.