It could have to deal with scarcity, as time that is scarce is perceived to be higher valued, as booking office hours with a professor, for example. But that's just my economics take on it.

Regardless of that, it still makes no sense to time-constrict a websites uptime. It's not as if websites run on manual labour! (well, it takes some to create one)

The irony is that a site that says "someone needs to come in and turn me on" must already be turned on. They've already written an app that has 100% uptime, they just use their uptime to claim the system is down.