A possible reason: Because appearances aren’t everything. I’ve seen multiple content-addressed distributed filesystems that planned to replace the web; I’ll certainly consider the possibility that this time around it might actually work, but I’m not holding my breath.
This is the key thing here. IPFS is for file storage; Syncthing is for file synchronisation. If you can usefully have a mountpoint for IPFS, then Syncthing should be able to use it for a directory. If you can’t, well, that’s a bit more complicated. That said, content-addressable file systems are extremely different from our usual ways of thinking, and I suspect that most of brig is actually dedicated to storing the metadata needed to build a file tree.
The one way I can think of that would make sense for Syncthing to do with IPFS would be if you could use Syncthing to synchronise things that aren’t files on your local filesystem - if there was a system through which you could have a node that stored its files on S3 (or, if you’re feeling particularly ironic, Dropbox). Then one of the ‘backends’ could use IPFS.
Mind you, by my understanding that would mean publishing the data so that it is accessible to anyone who gets hold of the IPFS address of the file. So probably not the best thing for private data, unless you store it all encrypted.