Sites can't be offered as torrents, because generally torrents are immutable once they have been created. You can't say "oops, that version of the file is wrong" and start offering the new version within that same torrent swarm.
However, there was an update to the protocol, BEP44, that did allow you to update the already-in-progress torrent.
Furthermore, there is a protocol called WebTorrent that swaps out some of the other base protocols for WebRTC, allowing a web browser to participate in the torrents. You could just include a link to the library via CDN. The trouble of course is that bittorrent now relies on DHT more and more (you wouldn't want to have to run a tracker, if you did it'd just be a target of legal attacks)... and WebTorrent can't do DHT (of any variety) well. There was a proposal to allow browsers to be able to do native network sockets, but I think that got turned down by Mozilla (maybe they were more concerned with doing VPN ads or something).
But if you had that, then yes, it might be possible to have something like a "swarmsite" that didn't need to be hosted.
See ZeroNet, which uses bittorrent to sync websites, including support for dynamic content and accounts. (Accounts are somewhat centralized last i checked) https://github.com/HelloZeroNet/ZeroNet
The main/original author disappeared but there are somewhat maintained community forks.
Torrents are immutable but they could just give us new torrents (with updated indexes) via chats/forums/etc. Then thanks to https://www.bittorrent.org/beps/bep_0038.html we can just download the diff.
However, there was an update to the protocol, BEP44, that did allow you to update the already-in-progress torrent.
Furthermore, there is a protocol called WebTorrent that swaps out some of the other base protocols for WebRTC, allowing a web browser to participate in the torrents. You could just include a link to the library via CDN. The trouble of course is that bittorrent now relies on DHT more and more (you wouldn't want to have to run a tracker, if you did it'd just be a target of legal attacks)... and WebTorrent can't do DHT (of any variety) well. There was a proposal to allow browsers to be able to do native network sockets, but I think that got turned down by Mozilla (maybe they were more concerned with doing VPN ads or something).
But if you had that, then yes, it might be possible to have something like a "swarmsite" that didn't need to be hosted.