In North America, why Medicine not an undergrad degree high schoolers can apply to directly? My kid showed me this New York Times article. Why don't Canada and US offer medicine as undergrad?

Australia does.


  If you want to go to medical school in Australia, you need to sit the UCAT in your final year of high school.


UK does.


  In the UK, it can take either 5 or 6 years for the undergraduate degree, then enrol on a 2 year foundation programme, during which they must complete work placements according to their specialisation, before applying to Medical Specialty Training. In total, then, medical studies in the UK can take up to 8 years.
  
  In Canada, medical study is only available at graduate level, so you must take 4 years of a pre-med undergraduate degree (although some institutions will accept only 2 years of this or offer a fast-track option), with the graduate medical studies taking another 4 years. After this, you must complete a residency training programme, which can take between 3 and 8 years, depending on your specialisation. Medical studies in Canada, then, take a minimum of 11 years.
  
  This is because the UK undergraduate qualification is equivalent to the combination of undergraduate and graduate degrees in Canada.
In Canada medicine is offered as an undergraduate degree, but graduates cannot practice until they have completed additional requirements (usually a residency, that can last between 2 to 6 years).  There is no BSc or HBSc in medicine that I know of.  

Although it is often touted as a doctoral degree and the graduates can use the title Doctor, the full title is Doctor of Medicine or MD and it is not a doctoral degree in the sense of a PhD (or similar) in other disciplines.  Several other health-related professions can also use the title of Doctor (optometrists, veterinarians), although again they do not necessarily hold PhDs (some do).

In Canada education is a provincial jurisdiction so requirements for admissions vary from one province to the next.  The common factor to all programs is the huge demand: there is often 10 or more applicants for every admission slot.

In Quebec you can apply straight out of usual 2-year pre-university (CEGEP) studies required for admission to most university undergraduate programs.   In Ontario your odds of getting in are next to none unless you have completed 3 full years of an undergraduate university degree, often in a cognate field: this is a simple way of cutting down on the number of applications.  It may well be that some schools require an undergraduate degree for application but that would only be for weeding purposes.

These days, given the extremely high demand in some provinces, an increasing number  of candidates are accepted after an advanced MSc degree: even an outstanding full undergraduate degree is no longer a guarantee of getting in.