So they have to leave their laptop plugged in and connected to the network - I think most Macs sold are laptops - which is a pain in general (prevents you from just closing the lid and forgetting about the laptop whenever you want), but especially difficult in various travel-related situations - say, one has brought the laptop with them but has left it in the hotel safe (or the hotel has poor Wi-Fi, etc.)... [1]
...and they have to be able to traverse the NAT, which is possible in some way on most, but not all connections (suppose Apple could proxy, but then you get back to data centers)...
...and they have to have good upload speed, if the browsing process from the client phone is to be sufficiently interactive, which, sadly, is relatively uncommon even on connections with good download speed, not to mention the many connections that are just crap...
...and if the server's drive fails, their treasured data is lost, so they should make sure to keep backup drives around (or online backups, but ditto data centers)...
...and they have to have a Mac (or PC) to act as a server, which an increasing number of iOS users don't (yet they still want to be able to store more photos than fit on their phone)...
For what? Saving a few dollars on a cloud plan for your expensive iOS devices? Preventing the NSA from snooping on your vacation photos? (Most people don't care, but for those who do, an easier solution for Apple would be implementing client side encryption. Of course, this does not currently exist, except for iCloud Keychain.) Saving energy? (Apple's data centers use purely renewable energy, your home probably doesn't.)
Reliability, resilience to cloud outages? That's a fair point, and an option to use a desktop as a backup would be nice, but I guarantee you most users would still opt to use Apple's secret mega-data centers in the desert. (Also, Apple's servers are more reliable than they were in the past, and since we're talking about new engineering effort - if they were as reliable as, say, Google, would this still really be an issue?)
The logic causing you to use a RPi makes sense to you (and I bet part of it is the flexibility that's available to a sysadmin/programmer but wouldn't be to an average user in any scenario), but for most people's use cases, I think it simply doesn't make any sense. So it's no surprise that Apple isn't working on it.
[1] A separate device like your Raspberry Pi, or Apple's AirPort Express, would solve this problem, but not the others.