The liability issue is a red herring of more recent days.
The simple answer is because they can.
There is an additional cost in obtaining the necessaary approvals and in maintaining the paperwork trail for real auditability. If you look at the cost in the US of TSO'd versus non- TSO'd instruments you will see the realisitic cost of the paperwork.
In the UK the whole thing is driven by people making a living out of a small market and pricing parts up to a level that will support that living.
As most CAA activity is centred around paperwork it allows people who write it to charge what the trade will bear.
Maintaining an old Auster where virtually all the remaining spares are ex-military and hence never had any paperwork is a real education!