I suspect people selling petrol to uninsured (keyless) drivers will expect a huge return on their investment so possibly making it cheaper in the long run to just get insurance?
Many uninsured drivers are already disqualified or have never held a driving licence so getting insurance isn't an option.
And why aren't car details scrutinised more closely? OK, there are millions of vehicles out there but computers have the capacity to check 100s of ANPR each minute - so how comes uninsured cars can even be around? As soon as insurance expires, a red flag should be raised on a central database (which I assume police, DVLA and insurance all subscribe to), so why isn't a local forced immediately informed of an uninsured vehicle registered on their patch? OK, so it takes people to go round there but why retire good officers early when they could be put to use for this sort of thing - especially if fines were set so high as to pay wages?
Firstly a huge number of uninsured vehicles are not registered to anybody that actually exists so tracing the registered keeper is nigh on impossible, secondly, and assuming you find the registered keeper, you then need to prove who was driving the car at the time, lastly the courts can fine people but can't make them pay the fine, they can of course imprison them for non payment but this actually costs the taxpayer more than the outstanding fine.