One possibility is that the Mechanicum deliberately preserves a weakness in case its alliance with the Imperium fails. An exposed stern might also help loyal ships destroy vessels that retreat, disgrace themselves, or fall to Chaos. A more practical defence is that ships can manoeuvre or use their thruster wash against pursuers. These explanations fit the setting, but a multi-kilometre warship still needs a way to deter attacks from the rear.
Notable Point · 9:16-10:16
Why might Imperial sterns stay unarmed?
Several in-universe explanations account for the fleet's aft blind spots, though none fully defends the design.