There is not a single insured nuclear power station, yet 449ish of them are working around the globe...
Meaning the owning authority have chosen to self insure and they only have to answer to themselves as, hopefully, their power station is not going anywhere that could involve third parties who might insist on a very high level of third party cover.
It won't be the hull insurance cover that is the issue with a pilotless aircraft, it will be the passenger and particularly third party liabilities cover that other countries/airports etc. will insist upon before over flight or landing rights are granted. Not sure about today but the benchmark for assessing the level of cover required was two jumbos in a mid-air collision over the CBD of a major city, it runs to billions of dollars.