My site being down for a couple days is not an unacceptably large loss, unlike an uninsured car being wrecked.
It also isn't a good analogy because insurance doesn't apply retroactively to wrecks that happened before start of term, and is event-based rather than providing continuous value.
I thought that's why it's a good analogy - DDoS protection doesn't apply retroactively to prior attacks (or even current attacks, it's hard to apply DDoS protection while your site is down due to DDoS). If you want protection from DDoS, you need it before the DDoS. If you want to insure your car in case of accident, you need to insure it before the accident.
It also isn't a good analogy because insurance doesn't apply retroactively to wrecks that happened before start of term, and is event-based rather than providing continuous value.