Because your service neither could handle a million concurrent users tomorrow nor a power outage. Those probably aren't your requirements, but they are for most companies.
A power outage can be dealt just like in the old days with a UPS unit.
SRE here.
92% of all companies can't handle this either.
sure, but 99% of sysadmins/devops/SRE/whatever will never work on anything that has million concurrent users.
Most startup will never reach million concurrent users. And if they do, investors will happily shuffle as much money as you need to make your site work at that scale.
Hell even million monthly users is a nice milestone that most projects never reach, and that usually translates to couple of thousand concurrent request (peak), that average laptop could handle.
A lot of things I see with millions of concurrent users aren't actually monoliths: Sure, Facebook needs to handle that. But most cloud apps would better be run where each tenant/business/team operates an Intel NUC in their basement, instead of the developer using the cloud as a way to force rent-seeking behavior.
And if your whole region has a problem, which is more likley to happen than one might think, then you want a multi-region setup e.g. us-west-1 and us-east-2, and then we can start to calculate the numbers of nines, unless your username is ocdtrekkie, he can beat AWS with a single NUC while he is sleeping.