A better formed Q would be to ask why does the UK CAA, and almost every other European CAA, create plates which are drafted for A4, when they could have drafted them for A5 and produced plates which are directly usable in the cockpit.
The taxpayer subsidy is irrelevant, because the European CAAs do produce free approach plates. They just make them barely cockpit usable, by drafting them mostly for A4 and mostly showing an OCA and not an MDA, etc.
Why do they go most of the way and then stop when they could have done something usable with minimal extra effort?
I asked the CAA head of mapping (face to face) and his astonishing and arrogant reply was that the CAA is not in the business of competing with commercial data providers.
You can take that answer in different ways, one of which is that the CAAs are in bed with Jeppesen and protect Jepp's business - perhaps, in some cases, in return for Jepp paying them for the use of their "copyright". Certainly, Eurocontrol were for many years protecting Jepp's business, though, I am sure, without getting any money for doing it.