Two main reasons.
Well the group is very successful, hard to argue about that.
Reality is where licensed clubs like Penrith, Balmain and Cronulla who apparently don't have a debenture system with boards consisting of supposed professionals got themselves into all manner of strife financially........and fell back on excuses re government regulations for their poor risk management the Wests group/HBG have thrived and grown.
Most of the debenture holders are wealthy individuals with sound business nous.
That's the financial reason.
Secondly, if you asked me 20 years ago I would've been less inclined to allow the system to continue.
However, the Wests group conceded much with the adoption of the HBG structure once Balmain Leagues was absorbed following their failure. Rather than as probably would have happened in reverse Wests group depersonalised their structure somewhat in order to be inclusive for all WT fans former Balmain fans. Let's face it in a few years maybe a Balmain Leagues will be built by HBG on the former Rozelle site.
So in some ways the debenture system which is really only a form of privileged membership which vet new members to make certain their values align with the original values of the Wests group guaranteeing the values aren't eroded further.