This question seems to come up fairly frequently.
When my wife started her lease we enquired about a Dashcam from both Motability and the Toyota dealer we were getting the car from. We wanted the hard wired, dealer fitted Toyota DashCam.
Motability’s attitude on the phone was ‘we don’t care’; the Toyota dealer checked with Motability – at our insistence – and got the same response.
It seems to be a non-issue as far as our experience goes.
As for why cars aren’t fitted with Dashcams as standard – they are illegal in some European countries (and UK drivers with them should be aware of the rules if going on holiday).
For example : they’re completely outlawed in Austria, Luxembourg & Portugal; Effectively banned in Switzerland (with very complex laws); You cannot upload Dashcam video to the internet if it’s in France or Germany, plus you need to inform all the involved parties if you are using footage in an insurance claim.
So – manufacturers seem to play it safe, by leaving it up to the user to install & use at their own risk.