If you were to host the entire forum on a blockchain, every node would have to hold the blockchain. So not scaling horizontally, but instead copying the “database” a bunch of times. Think of hosting all of the data in reddit on a thousand nodes. Sure you could access it from any node, but the database would be just as big as before, just copied around a bunch of times.
In a way this thing is already much more decentralized than a blockchain could ever be, in that every server doesn’t hold all of the data at once. Much better use of resources IMO.