Here in Japan, car insurance with unlimited/unlimited liability for person/property is pretty basic. It is considered that why unlimited is important is because normal people isn't good to estimate how much max liability is needed. Anyway no one would abuse unlimited insurance (but may abuse within limited range) so set it unlimited won't up insurance fee much. IIRC Some insurance also offer something like 50M JPY limited plan but it just save about 2k JPY per year (YMMV). Anyway, the point of insurance is to cover very rare unpayable reparation event, so why not unlimited?
I guess insurers want to avoid being bankrupted by a single disaster, and prefer that the insured person goes bankrupt instead. Unclear what is the socially optimal policy, but I'd anyway assume if a car accident ever causes like $1B in damages, government will end up paying the bill.
