curl currently looks on Linux and othe unixes at the file ~/.curlrc for its default --config file. Clutter directly in $HOME is considered by many users to be undesirable. It makes it hard to distinguish between documents, configuration, and caches, complicates e.g. "dotfiles" repositories, and makes it hard to nuke cache data that can be safely deleted at the cost of making the program work to recreate it.
curl also reads ~/.ssh/known_hosts but similar as for the .netrc, that's using a file typically written/used by something else: (open)ssh. It probably makes sense to not use $XDG_CONFIG_HOME for that either.
You certainly cannot do any such thing unless ssh decides to support that first.
(It does not, and will not -- the openssh developers perspective on this is that root needs to know where the ssh configuration is, and therefore it cannot be configurable via an environment variable.)