Because Ofcom do not allow this. VDSL is "notched" to not use or at least use much lower power on the ADSL frequencies because otherwise it would put Sky, Talktalk, etc at a disadvantage by causing interference with their exchange based ADSL2.
If BT supplied you with ADSL from the cabinet, Talktalk's investment in a DSLAM at the exchange would not be able to compete.
It's all about the false competition we have in UK broadband.
Were this to all be binned, you'd get a much better experience on VDSL as it would use the ADSL frequencies as well from a much lower distance.
In fact, the cabinet DSLAM almost certainly has the technical ability to provide you ADSL2, this is just not allowed in the UK.
Thanks! That really answers my technical question, if negatively. It is a shame because the chance of anyone unbundling anything at our exchange is zero. Even if there was any money to be made from our approx 1000 subscribers, the fibre enabling with Welsh government money has creamed off all the accessible and many of the inaccessible customers.