We do try to develop Instructors internally. But there is a bit of a gulf between the level of experience we require to train CPLME and IR candidates and what we can turn out through internal training. The Instructors need the time on twins and IFR that just can't be achieved internally.
EASA may allow an Instructor to teach ME after just 30hrs. But that does not provide the experience we seek or the student deserves. You only have to look at the newly qualified ME Instructor that let a student wheels up a Seneca at the big school next door recently. That sort of event would be catastrophic for us.
The same goes for IR training, students doing intensive IR training needs an Instructor who knows there stuff inside and out. A basic IRI(A) on an Instructor who has not been flying real world IFR just does not cut it.
Its the irony of the industry using the most inexperienced to train the most inexperienced......