Wouldn't you also need two load balancers then? Otherwise you've still got a single point of failure. And how do you keep the failover system in sync? It's a whole can of worms to promise 100% uptime. It's super rare that a server physically breaks and suddenly goes dark with less than a day's warning, and for most applications such a once-in-5-years event is tolerable if that means the hosting costs are divided by five as well (so far I'm ~10 years in and haven't experienced such an event yet; disks in RAID have broken but not a cpu or other SPOF)