it mean English people doubt other english people. why canadians ask uk people to sit for ielts? don't they trust people from uk can also speak english?
it's not all Speaking English... Reading comprehension, Writing correct grammar and spelling, Listening and understanding English language without difficulty. It's also about being fair to everybody else on having uniform requirement. Personally, It will be unfair to me if I'm required to submit English profiency test results and those English-speaking country (applicants) are not required anymore.
in my humble opinion only and just my two cents.