For the same reason why they need a missile that can outright destroy a Supercarrier in one hit. It's just all about insurance. Just like when you require an insurance against burglars even though you live in a safe neighbourhood but what if...?
Ishihara is just your average right wing bigmouth who tries to make a name for himself like his predecessor, Koizumi did back in the early 2005. As you guys might know, Koizumi made himself look good to the western hemisphere (especially with the US) while antagonizing China, Russia and both...