The reason is because there’s a phenomenal amount of money to be made in telecommunications, and because of the legal situation that the US Government foisted upon AT&T, pretty much anyone can step up to claim their share of the pie and AT&T has to play nice with them.
It’s informative to look at a map of trunk lines. You rapidly realize that with few exceptions (Sprint and MCI have some lines but, last time I saw a map, nowhere near a nationwide network, and there were a few other minor local players), anyone selling long distance service is buying it “in bulk” from someone else (usually AT&T) and reselling it to you.
Telecommunications is an area with a high Fear, Uncertainty, and Doubt factor. In the past couple of years alone, I’ve heard of companies that:
Register a name which SOUNDS like one of the Big Three, so that their operators can “mishear” your long distance preference and route you via their network when calling from a pay phone (at exorbitant rates, natch);
Register names that aren’t really names per se, but phrases that a person might give in response to the question “What long distance company would you like to use?” such as “Oh, it really doesn’t matter”, again so that their operators can route you via their network and then blame you for requesting it if you complain;
Use confusingly worded contest entry forms as authorization to switch your long distance provider to their own network;
Switch you without any authorization whatsoever and hope you won’t notice and/or complain (“slamming”);
Register names which are extremely generic so that the immediately preceding two tactics are not as glaringly obvious on your itemized long distance bill;
Attempt to submit spurious charges through your local telephone billing agency (“cramming”) and hope you don’t notice or complain.