Unfortunately you are woefully ignorant about eating disorders. They are not "mostly psychological in nature" (whatever that means--is that like being "mostly pregnant"??). They are BIOLOGICAL illnesses that are largely heritable. They are not "about" psychological issues.
Please refrain from making pronouncements on subjects you know nothing about.
As for prevalence, the numbers have been steady for as long as they've been recorded. There is likely an increase in diagnosis and treatment now.