Because you want syncthing to delete files more often than not. When you want the reverse, it's a single property on the shared folder (ignoreDelete), feel free to set it.
You can already build such a system yourself quite trivially by getting an FTP account, mounting it locally with curlftpfs, and then using SVN or CVS on the mounted filesystem.
Anyways, if you're at the grocery store and you need to see your shopping list, and you're happy with your current workflow of opening up Termux and typing `rsync -avrsomgwtfbbq me@server/shopping-list.txt` with your thumbs, then by all means keep doing that! For the rest of us, there's Syncthing.
If you really want to be fancy, you can deploy a script to watch files for changes and do rsync immediately when it happene.