I think the cabinets are usually pretty full. They may also not have appropriate racking (or whatever) to support the additional hardware. I suspect if BT could do it then they would as adding additional pods will cost more.
My fibre cabinet is about 80m away from the PCP. This is admittedly quite extreme: but if they were ever to deploy G.fast here, putting it on the fibre cabinet would burn a good chunk of the link budget.