That's because decentralized networks are expensive and can't handle spam unless you make receiving messages opt-in, and then you can't @ people like you can on Twitter.
We had, and still have, standards to deal with crossplatform messaging, like Jabber or Matrix.
What prevents that from catching on at scale is, the "big boys", like MS, FB or Google, mostly not playing ball and never implementing these in their own messaging platforms, to keep their gardens neatly walled from each other.
As intraplatform exchange is not really in-line with what most of these platforms are striving for these days; Interactions with their own platforms and the advertisers on it.
