The key here is that that is for a SLA. So it's not that they'll never be down, but rather for every minute they are down they'll pay out money to you for the trouble.
To extend the car analogy, it's more like a lifetime warranty on a car: the car might break down all the time, but its still covered.
They're explicitly not saying that it will never go down. They're saying that if it ever does go down, they will pay their customers a penalty. Without having any "acceptable" amount of downtime that they don't have to pay a penalty for.
Really, too-many-9s SLAs are what's ridiculous. So you say you "guarantee" 99.99% uptime, but only interruptions of 30min or more actually count? 2 incidents of 29 minutes would put you over that .001% that you claim, without either counting.
I would definitely buy a car if they advertised it to never break down - if the compensation on the promise would be good enough!
But seriously speaking, a 99.99% SLA is not more realistic or achievable in itself than any other SLA promise. It all depends on the provider (how they strive to reach that promise) and what they promise in return if the promise is broken.
Nobody claims they will give you 100% uptime. The service level agreement is at 100%, that just means that the service should be up 100%; when it isn't, you get compensated. They're saying that any percentage of downtime (modulo time limits) is bad, not that it won't happen.
To extend the car analogy, it's more like a lifetime warranty on a car: the car might break down all the time, but its still covered.