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

[Ömer Sinan Ağacan]

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?