I'm totally not justifying this model, but I can kind of understand where it comes from. ATO = freelance, on-call work for most of the time. There are students, you work and get paid by the hour. There are none, you stay put at home. You don't have any long-term contractual obligation towards them so that they're interested in investing into your qualification. And, while waiting for another course, you might just as well find work elsewhere.
That's the reason. In airlines, soon-to-be instructors usually have their training paid for by the airline, but are also made to sign a training bond for 2-3 years of service after obtaining the qualification. In ATOs, they either find freelancers from the airline world or someone willing to self-fund. This applies to pretty much any instructor ticket you can think of.