1. Companies don't know how to do the engineering for maximum uptime, like you describe. It's way more complicated than the usual CRUD operations
2. Companies know how to but they decide not to invest this time (we often traded one hour of downtime against 2-3 man-days for preparing online schema changes with nasty and inconsistent backfilling in the early days).
And 3. Don't forget disaster recovery. I've seen some of the smartest companies go down for hours due to a DB misconfiguration, or a Rack PSU faulting with only one side of the servers connected, even with a reasonably highly available setup. Stuff like this happens - and then you better have a proper 503 Maintenance page up and running to prevent Google from delisting your site. In this case though, "maintenance" is rather an euphemism :)
Companies know how to but they decide not to invest this time
Cost increases exponentially for diminishing returns once you get into serious availability. For most businesses, the investment in moving from 99.9% to 99.99% or 99.99% to 99.999% uptime just isn't worth it - most customers are quite willing to "try again later" in practice, especially if you give them advance notice or have a regular maintenance slot.
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There's more and more nuanced reasons actually:
1. Companies don't know how to do the engineering for maximum uptime, like you describe. It's way more complicated than the usual CRUD operations
2. Companies know how to but they decide not to invest this time (we often traded one hour of downtime against 2-3 man-days for preparing online schema changes with nasty and inconsistent backfilling in the early days).
And 3. Don't forget disaster recovery. I've seen some of the smartest companies go down for hours due to a DB misconfiguration, or a Rack PSU faulting with only one side of the servers connected, even with a reasonably highly available setup. Stuff like this happens - and then you better have a proper 503 Maintenance page up and running to prevent Google from delisting your site. In this case though, "maintenance" is rather an euphemism :)
Companies know how to but they decide not to invest this time
Cost increases exponentially for diminishing returns once you get into serious availability. For most businesses, the investment in moving from 99.9% to 99.99% or 99.99% to 99.999% uptime just isn't worth it - most customers are quite willing to "try again later" in practice, especially if you give them advance notice or have a regular maintenance slot.