It's not cheap to have any confidence of any uptime realistically at all. The thing is that most people either live without that guarantee, or just get lucky enough not to care. It becomes problematic if you've made promises to others about uptime that are built on a house of sand.
Unless your base load cloud costs are more than the cost of full time, ready at a moments notice, experienced ops people you don't get close to any guarantee of uptime by non-managed hosting. The salary cost alone of that is substantial, let alone hardware spread across multiple locations. My firm pays at least 7 figures a year on IT ops and don't come close to 99.9% uptime across everything.