If you're served from a cabinet you should be being supplied wholesale UBA internet because ULL connections via a cabinet introduces midspan injection issues which will hamper speeds. This isn't a Telecom/Chorus problem, it's the fact ISP's chose only to install equipment into exchanges and not cabinets.
As far as I'm aware it's not a "choice" that ISPs put their own equipment in cabinets - they simply cannot.
There is nothing at all stopping an ISP or telco doing that. It was a basic requirement of the cabinetisation project to allow space for other access seekers.
The problem is the cost of a) equipment and b) backhual costs (which are set by the Commerce Commission).
With each cabinet typically serving a maximum of 288 properties an ISP that had 10% of the market might capture 15 users. There is no way to build a business model delivering a ROI with such a small number of users when hardware costs as well as backhaul costs from the cabinet have to be paid for and you're potentially getting a few hundred $ per month from the combined customer base from that cabinet.