Because there are lenses in the headset that focus the two images at a set focal distant (between a couple of meters and infinity focus, depending on the headse

Because there are lenses in the headset that focus the two images at a set focal distant (between a couple of meters and infinity focus, depending on the headset). You need glasses for the same reason one would need glasses to see a flat image several meters aways, or to see the stars as the case may be.