Doctors are humans too. Depending on where they get their education, how long they do their training, how their patients are prioritized on certain days, and believe it or not, how much sleep they get on certain days, they can't properly diagnose everything, simply because they just don't know everything (especially when they're ambiguously asked about a random spot on your face.) So be grateful that what you had wasn't anything serious, and that you can actually afford a healthcare that so many other people can't.