Different systems and different operations, and not enough data being shared between them.
The station announcements are plugged into GO's train scheduling system, not their dispatching system. They know when their trains are on time or late, and can broadcast warnings and changes of that. They can even broadcast platform changes.
But the scheduling system does not display VIA trains on it (it's not a graphical interface), and so they don't know when and where the VIA trains are. Thus, they can't broadcast warnings for when their trains are about to pass.
Dan