The liquid diet is first and foremost to reduce the size of your liver, so that it can be easily moved aside during the surgery. According to my reading materials, it's also to promote rapid weight loss, and to soften up the belly fat. I also feel that it's good practice for my mind having to learn that I won't be putting that much food in my mouth again.
I think you might be in danger of having your surgery cancelled if you can't be compliant with the liquid diet. I am in day 2 of that, and it IS tough! BUT - if I can't follow this regime for 2 weeks BEFORE my surgery, how would I ever be successful for a lifetime AFTER my surgery?