Well, (a) why would they? (b) "uptime" has shifted from a binary "site up/down" to "degraded performance", which itself indicates improvements to uptime since we're both pickier and more precise.

Are we really questioning why cloud providers would offer better uptime guarantees?

Yes, I'm asking why they'd lock themselves into a contract around 5 9s of uptime since the parent poster mentioned that they won't do so. Of course, AWS actually does do this in some cases and they guarantee 99.99% for most things, so it feels a bit arbitrary - 5 minutes vs an hour, roughly.

So then its clearly not as trivial to achieve as you made it sound.

Are you replying to the right person?

You can certainly sign a contract for five nines SLA with cloud providers.

You just won't like the price.

Then it’s clearly higher risk?

If you are asking this question you don't understand what it takes to hit 5 nines in a real life measured system.