Because "uptime" and "nines" became a marketing term. Simple as that. But the problem is that any public-facing measure of availability becomes a defacto marketing term.

The older I get the more I hate marketers. The whole field stands on the back of war-time propaganda research and it sure feels like it's the cause of so much rot in society.

Also 4-5 nines is virtually impossible for complex systems, so the sort of responsible people who could make 3 nines true begin to check out, and now you've getting most of your info from the delusional, and you're lucky if you manage 2 objective nines.