Setting aside monoplies casued by, or abetted by, regulation (e.g. utility companies), competiveness of markets can be modeled as a function of barriers to entry and barriers to customers switching.
if it’s very hard / expensive for somebody to enter a market then that’s a relatively uncompetitive market. For example, to try to duplicate FedEx’s domestic US service today from scratch would be prohibitive. Fedex does have competitors, but they’re not nearly as worried about new entrants as is, say, United Airlines, even though both are in the transport-stuff-from-here-to-there-by-air business.
The second point is a little more subtle. At the first level you’ve got the question of how easy is it for customers to switch to a direct competitor. If XYZ Corp uses Fedex, how hard is it for them to switch to UPS? Not very, unless they’re a big business (say Land’ End) with a lot of deeply embedded interconnections between Fedex & my processes.
The real meat is at the second level: how easily can customers stop using any product like mine? Fedex started out making most of its income from carrying documents from one law firm or government agency to another. A huge slice of that traffic is now electronic.
Fedex’es mix of paper-based cargo versus tangible goods has gone from 80/20 to 20/80 over the years. Email is trmendous competition for the paper, and zero competition for the tangible goods.
Finally, competitiveness of markets is a spectrum, from highly competitve to almost wholly uncompetitve. People make a very basic error in thinking which clouds the rest of the argument, when they take a full-spectrum value and unconsciously convert it to a binary value, “competitve” or “uncompetitive”.
Having said all that, there are many largely uncompetitve markets out there, where one or two incumbents have the market to themselves and enjoy fat margins. The situation is not long-term stable, but it can last for many many years. This company http://www.pelco.com/ , for example, has a near-monopoly on supplying outdoor security cameras in the USA.
At a local level you can get de-facto monopolies such as the only restuarant in a small town or the only road construction company in a county. They’re somewhat price-constrained by the customer’s knowledge of competitor’s prices not too far away.
Others, like Pelco, remain almost alone since the national market isn’t large enough to justify a conglomerate moving in, but it’s too big for a start-up to get enough financing to break open.