Another question about -XStrict: Why not implement it as a Core pass?

So this is another question comes to mind. It seems to me like it would be a lot easier to implement, we could even implement it as a plugin, without changing anything in GHC. (I mean -XStrict, not -XStrictData) I'm wondering why it's currently implemented on Haskell syntax. Any ideas? Is it because of some typechecking related things?