They are doing that (see their project glasswing over the past few months), but there's a lot more code in the world than you realise.

The problem with rolling it out is that bad and good actors can both use it at the same time, and bad actors will typically move faster than typical day-to-day software projects and patching schedules, so they set up glasswing to give access to the major producers and projects to patch their own software before it becomes available more widely (they've submitted tremendous numbers of security issues to open source projects)