They do when it makes sense. Hosting small Giles is not a big deal, and small files would be a bad experience on torrent, since by the time you get peers to start the download you would have already finished it from a normal server, not to mention that you need to host the torrent file anyways. Also things that have lots of releases/patches are a bad experience, since people might be seeding an old version.

However, large files that only get sporadic releases, such as distros iso, can definitely benefit from it and you can usually find torrent links for them.