Because it matters to the end user that all the instances are cross compatible, that's the federated part. When I first heard of Lemmy and Mastadon as "self hosted social media", I assumed that all the instances were isolated, and dismissed it as pointless. Once I learned what federation was, possibly through the email analogy, I was instantly onboard.

We're not at a stage where you can make full use of these platforms without having a basic understanding of how they work. A disinterested idiot is going to go " WTF is an instance, why is [whatever instance they landed on] so empty" and give up. The email analogy is useful for the interested skeptic and they're the people that are most likely to stick around.

In this thread the email analogy has been criticized for being not technically accurate enough and too technically accurate. That suggests it's about right.