There are some areas of western allopathic (and other types of) medicine where research hasn't progressed very far, and practitioners actually don't have much knowledge, or solutions.
The doctors are doing their best, or at least doing what is expected of them, by going through the main accepted causes and treatments one at a time to try to help you, but in murky areas like this strange lip condition, they are often just guessing and hoping something will take effect.
These days, with the availability of information to the public's being greater than ever before, sufferers of a medical condition, if they spend a lot of time researching it, can be better informed about it than a general practitioner doctor. I'm not saying they would understand all the chemistry and biology and connectivity to the rest of the body as well as a medical professional, but they might have more up-to-date and specific knowledge of the exact condition.
Many past forum postings here mention how dermatologists seem to be surprisingly ineffectual at helping sufferers deal with cheilitis, and I think there was even a member here whose brother was a doctor (and so he would tell his brother what to prescribe for him, and got it without fuss) who said that doctors are generally pretty unhelpful about this condition.
One lesson that life has never wavered on teaching me is not to give over to "authorities" any expectation that they would have greater knowledge, concern, ethics, responsibility, trustworthiness, etc. You have to be your own protector, researcher, defender, philosopher, etc.