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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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.