I mean, there is something seriously wrong with this system where instructors are only hired by the school that trained them? Really?
The simple fact is that schools know that jobs are scarce, and highly valued. They also know that they make quite a bit of money from an Instructor Rating. If they have a job position available, they know that they can use that to sell an Instructor Rating, and thus make a bit of cash from that job offer. That's why they see giving that job to someone who already has an Instructor Rating (and no experience, so no value to add in the form of training or testing approvals) as a missed opportunity for them to sell a rating course.
So, yes, your Instructor Rating is a standardised rating. But you're not going to get a job at any place that offers Instructor Rating courses, unless there's a bit of a boom.
And yes, it's always been that way - I did my rating back in 1989, and it was pretty well known then that you didn't do a rating without an offer to work (even if only as a casual) at the end of it. Your school should have known that.