Why don't people tip extra when there's an automatic gratuity included in their check?

I work as a server in a fine dining restaurant that automatically adds an 18% gratuity to every check. Here’s my issue: Aside from being pretty, charming, hospitable and knowledgable, I happen to be an extremely efficient and attentive server. So why is it that most people seldomly choose to give me additional gratuity when it comes to paying their check at the end of their meal?
I mean I can tell they loved the food, the service and the entire experience, and at the bottom of their check where there is a very visible spot that reads:
Additional Gratuity:________________
More often than not I will find a big slash occupying the spot where a dollar sign followed by series of digits should be.
Personally, I’m accustomed to tipping at least 20% even for unremarkable service, as long as nothing goes wrong, and I’m talking about when I just go to your typical T.G.I Friday’s. When I go out to a fine dining establishment, I automatically increase my standard 20% to at about 30% and this is me….a server, not some loaded black american express member.
Are people just plain cheap or do they purposely not tip extra as a way of capping the earning potential of a server?