Because if you can easily file off the serial numbers and make it a non-Trek story, then was it a "true" Star Trek story in the first place? Certain settings are unique, such as the Trek universe, and it lets writers explore ideas and story lines dependent on the dynamics already set up there. Yes, I suppose I could go back and file the serial numbers off my Trek stories, but that would entail adding in a lot of exposition and history for this new story universe, stuff that already exists in the Trek realm. If you're going to use the Trek universe only as a setting and not make the story truly "Trek," then you really are using a crutch. Why not create your own setting, if the setting isn't important?