That's not always true. For example: Once-upon-a-time there were three (3) major U.S. long-distance carriers, some of which were also CLECs (Competitive Local Exchange Carriers) in some markets: AT&T (no, not today's "at&t," but the real AT&T), WorldCom (aka: MCI) and Sprint. At the time, competition for long distance subscribers was fierce. Instead of doing the intelligent thing, offering the best service they could at the lowest price point at which they could make money, they engaged in a constant battle of undercutting one another until none of them were making money. AT&T and WorldCom/MCI are gone (AT&T was Borged by SBC, WCOM/MCI by Verizon) and Sprint, I believe, has divested itself of all of its LD operations.
The point here isn't that landline LD is dead. That would've more-or-less happened, anyway. The point is the companies that were once very big in those markets essentially killed themselves by trying to be too competitive.
The airline industry, since deregulation, is sufferering the same fate. Once-upon-a-time, passenger air carriers competed as much on quality of service as they did price--if not more so. Now? Whomever can sell seats the cheapest--which also means whomever can jam the most seats into the available space. Now air travel more resembles being transported by high-speed cattle car than the relatively pleasant experience it used to be.