ANs are not very rare. They are diagnosed in 1 ~ 2 people per 100,000 people per year. That "per year" portion is important to realise. If you do the maths, for an average longevity of ~75 years, it comes down to 1 per 1,000 people are diagnosed with an AN in their lifetime.
BTW, 90% of people with an AN are never diagnosed.