Fediverse

Right, the internet is just a network that more complex things like social networks are built on top of. Hence why the question is with the design of Mastodon and completely unrelated to how the internet works fundamentally.

And I didn't say Mastodon actively prevents propagation, rather that some design decisions are at odds with viral propagation. It could be possible to build separate tooling on top, or to create forks that are more friendly towards propagation. However, Mastodon is by far the biggest ActivityPub based network right now, and the way it works plays a huge role for how Fediverse can be used at the moment.