Because like I said, it doesn't mean what you're saying and in debates it never does. You're the first person I've ever heard say that in my decade here, and I've had this discussion a billion times.
The reason it's convenient is because it's such a mealy-mouthed (and almost certainly incorrect interpretation of its use) that it can be twisted to mean anything to anyone. What makes the competition worthy? Why does there have to be a second viable option if most people dislike it? What is the point of even saying it?
Whereas when someone says "competition is good" in the traditional interpretation - which is everyone else but you - at least it means something. They're arguing for the opposite of a free market system, but they want that because they believe competition breathing down each other's necks motivates each other. There's at least some viable arguments to be had there.
I think you're off base. If you don't understand why someone saying "competition is good" would mean it's good to have options, and it tends to mean the more chances for good offerings the better, I can't help you.