need file extension in url to access template
The documentation for how Craft picks a template is here. Two of the relevant steps are:


  
  Is it an entry/category request?
  If the URI matches an entry’s or category’s URI, the section’s/category group’s template will get loaded, and the matched element will be made available to the template via a pre-populated entry or category variable.
  


and


  
  Does the URI match a template?
  Finally, Craft will see if the URI is a valid template path. If it is, it will return the matched template.
  


/about/our-people matches #4, and you get the about/_entry.html template. /about/our-people.html matches #6, so you get the about/our-people.html template.

The important thing is that Craft uses the first rule that matches.