For the most part, it's about EV efficiency. Because its HP and SpD are already so naturally high, investing in those doesn't have as much return on investment. Capping your Defense at 252 Bold actually more than doubles your ability to take random physical hits. Think things like U-Turn from a Dragapult for example:
- 0- Atk Dragapult U-turn vs. 252 HP / 252+ Def Blissey: 96-114 (13.4 - 15.9%)
- 0- Atk Dragapult U-turn vs. 252 HP / 0 Def Blissey: 222-262 (31 - 36.6%)
That's a roughly 21% difference in damage at max rolls. Meanwhile, here is the difference for Special Attacks with the same Dragapult:
- 252+ SpA Dragapult Draco Meteor vs. 252 HP / 0 SpD Blissey: 151-178 (21.1 - 24.9%)
- 252+ SpA Dragapult Draco Meteor vs. 252 HP / 252+ SpD Blissey: 114-135 (15.9 - 18.9%)
That's only about 6% difference at max rolls. While you are right and you never intend Bliss to take physical hits, it might on occasion through a battle, and EVs are far and away more efficient invested there. If not, something like U-turn chip could wreck your otherwise solid counter.