So to summarize: a good version of WebMD would have:
    * structured, quantitative information
    * real-time updates
    * summaries of supporting evidence
  
  So why hasn't anyone done this yet?

They have: https://www.clinicalkey.com/ However, it's for professionals, and it's not free. Only a professional has the knowledge, experience and context to use the information properly. For patients there are separate products/resources that clinicians can use to give patients more information about their case. But the patient can't just diagnose themselves. And certainly the effort of curating the dataset and developing the toolkit to power it isn't free.