It removes the complexity of actually finding that "outbox", because it is as straightforward as DNS, imagine if every HTTP client needed to go giga brain on finding where every hyperlink is hosted, the web wouldn't scale.

More importantly you can tell people what "big" relay fo you trust besides the outbox, getting all the scalability of centralized few relays while maintaining the unilateral exit if that relay bans you, with much less disruption of hyperlinks resolvability 

 that's my point

it doesn't replace outbox, it can improve it, but it addresses a different problem 

 The question was, why people don't implement the outbox model, and the answer is, because it is too difficult and hard to control its quality. Whereas Pkarr does the tried and tested strategy of DNS, just with sovereign TLDs.

Of course designating a relay as your Outbox is smart, in fact most people call these datastores :)