Because EASA require an ATO to have a Training Manual for all courses it offers
But there is no Training Course
required for someone who holds a non-EASA ICAO IR and the relevant experience. The Candidate can go straight to an Examiner and take the Skills Test (and oral TK Check).
As there is no 'Course' there is no need for a 'Course Approval'.
For safety reasons an ATO may want to check out a pilot before hiring an aircraft to them, but even that shouldn't be necessary as the Examiner (who, presumably, they would know) would be PIC.