Because those that ARE saved--whether they know it are not--are saved by and through the Catholic Church.
Let's remember the way it was in the early centuries. Heretics were never rebaptized when they reconciled with the Church, because their original Baptism was considered valid. If it was valid, it had its intended sacramental effect--which was to join that person to the One Holy Catholic Church.
Protestantism is in the exact same position. When you were baptized, you were baptized into not A church, but THE Church. You *became a Catholic* sacramentally if not organizationally.
This is how you harmonize the language of the Syllabus of Errors with Vatican II. So:
17. We may entertain at least a well-founded hope for the eternal salvation of all those who are in no manner in the true Church of Christ.
*In no manner* is the key language there. If a person really is Catholic in no manner, there really is no hope. If however, they are Catholic in some manner, there is hope--and this concept is what Vatican II sought to explore.