Importable STL Header Units?

[Zopolis0]

Currently, when importing the standard library, one has to
separately compile each unit they want to use, which is a hindrance to the
build process and a gap in the implementation.

Is there any particular reason why gcc doesn't provide importable header
units for the standard library? Is there a timeline for this? Given that
the standard library can already be compiled into header units, is there
any particular reason why those aren't shipped with gcc?