I suspect that the Borg don't generally place too much emphasis on a strong outer hull or structural integrity due to their reliance on shields and a distributed design of the ship. Remember that the E-D managed to do some pretty significant damage to the cube with a few phaser blasts on their first encounter before the Borg adapted.
Starfleet probably dedicates more effort into keeping the crew alive by making the outer hull stronger which served them better in the encounter with 8472.
Starfleet probably dedicates more effort into keeping the crew alive by making the outer hull stronger which served them better in the encounter with 8472.