Because you rented from Tocoo or other tour agencies or websites that only offer all-inclusive rates? Otherwise all Japanese rental companies offer optional "insurance".

Toyota: https://www.toyotarentacar.net/engli...ent/#insurance

"Deductible Compensation System fee: 1,080 yen (1,000 yen excluding tax)/day (24 hours)
By applying for this system, the customer is exempt from paying the above deductible compensation for damage liability in the event of an accident."


Nissan: https://nissan-rentacar.com/english/...insurance.html

"Collision Damage Waiver 1,296 yen (1,944 yen for E9・E10, 2,160 yen for M1, and 1,728 yen for T4・T5・T6)"

Times (Mazda): http://www.timescar-rental.com/support/

"Times Car Rental offers 3 types of insurance/coverage in order to support your trip. Basic Package, Safety Package and Super Safety Package. Tax is not included in the price. For each package, the customer will only pay up to 15 days for rentals within one month."

OTS: http://www.otsinternational.jp/otsrentacar/en/menseki/

"Okinawa area　1,620yen（All classes except Micro bus）　2,160yen（Class: Micro bus）
Hokkaido area　1,620yen（All classes）
...
We also have Safety Pack (540JPY/24hrs.) that compensate Non-operation charge."


As stipulated in the insurance document provided by some credit card companies, or if you call them to inquire, you will find that in order to get coverage from the credit card, you must decline rental agency's CDW or similar coverage. If no such wording exists on the contract, no box to check - such as in Japan, you can(must) write "I decline CDW" on the contract to get the credit card to cover the NOC in Japan.