I've spent a lot of time contemplating these ideas, as I have a personal interest in building my own friends/family intranet. Some challenges:
* How do you maintain and update software reliably without becoming a part time employee for maintaining your own personal infrastructure?
* How can we handle failure (such as hardware dying of old age) as gracefully as web services do, without the budget and infrastructure of a modern data center?
* If you're interested in detaching from the modern web, you probably care about sophisticated privacy and trust controls. Most easy-to-find open source software is not so sophisticated.
* Plugging into a lot of "modern" systems takes a lot of knowledge, effort, and sometimes money - DNS (dynamic DNS if your ISP won't offer a static IP), TLS, email, text, and push notifications, for example.
* How confident can you ever be about security? Ideally you want access to your service from the internet for trusted persons and devices - but the moment you expose your service outside the LAN, you're up against the world and a single vulnerable service can spell disaster.
* The most straightforward way to detach from the modern web is to yourself become a smaller, centralized service - a private facebook, if you will. But why would anyone want to take the effort of reuploading themselves and their private lives to your personal facebook? You own the data, they don't. If anything, their privacy feels more at risk, because a single person is more likely to fuck up security than the entirety of the facebook organization.
Edit:
Also, I think a large component is that there's "so much internet" in most peoples' lives, that I think many people hardly want more. Who wants another app, another web service, another way of sharing photos or videos or sending text messages?
A large part of the complacency of the modern web is that most people (that I can see) are tired of it, whether or not they realize. There's only so much information that the modern human needs, and I think if you've spent a few years on facebook you probably feel like you need less information, not more.