Surely it would be cost effective for openreach simply to tap a FTTC Cabinets fibre for FTTP?
For a single order maybe, for the future build cost of the network probably not.
It could also complicate the network in the future.
They strategically placed the Aggregation Nodes deliberately with a future FTTP rollout in mind, ideally both their location and coverage
An Aggregation Node can serve around 1400 homes with FTTP.
An Aggregation Node usually serves 3 or 4 FTTC cabinets, depending on the area.
Ideally the number of homes those cabinets serves is under 1400, that way the single Aggregation Node placed for FTTC all those years ago is all that's needed for FTTP in the future.
I'm most cases there aren't enough spare fibres at the FTTC cabinet to serve all the homes that cabinet covers.
It might be cheaper for a single FTTPoD order to source the fibre from an FTTC cabinet but OpenReach build the network for an FTTPoD order in the same way as they build native FTTP.
OpenReach won't want a mix of different routing for FTTPoD and FTTP lines.
In years to come it could be very confusing if random splitters were fed from completely different directions to every other splitter in the area because of an old FTTPoD order.
There's already going to be ducting between the Aggregation Node and the FTTC cabinet so it's usually an easy job to take the fibre that route, without touching the FTTC cabinet or its spare fibres.
OpenReach do actually take fibres from the FTTC cabinet for FTTP in some rural deployments.
They can also add a Subtended Head End (new hardware) to the FTTC cabinet and serve upto 1024 homes with FTTP from there.
These again aren't the standard build types (at the moment) so won't be how many (if any) FTTPoD orders are built.