Because it's not an EGR passage, it's where the hot air choke stove for the automatic choke operation bolts on.
The EGR is vertical, and behind the choke stove in this picture. This is a 1973 Firebird factory cast iron manifold. Edelbrock did have a cast aluminum plate to block that passage back in the late 70s/early 80s, I'm not certain if it's still available, or you ca take a piece of 1/4 inch steel, and fabricate one from scratch.