The most common reason owners skip cat insurance - 'she's indoors, what could happen?' - protects against the wrong category. Indoor life genuinely reduces accidents: traffic, fights, misadventure.
It does approximately nothing about illness, and illness is where feline veterinary spending concentrates. A cat that has never seen the street can still block its urinary tract at 2am, develop kidney disease at ten, or swallow the hair tie it spent three weeks stalking.
Indoor is a lifestyle, not a warranty.