It's because SE programmed the higher ranked NPCs to prioritize their item usage on you. The only time when they use items on each other is when they're grabbing a new power up.

It's a silly way to increase difficulty, sometimes it feels like it's a 7 vs 1 race. You'll see the AI in front of you REFUSING to use his Baccus UNTIL you pass him, even though he blatantly has 2-3 other NPCs in front of him. Not to mention they trigger AOE traps but hardly get hit by them.

The AI system needs a lot of tweaking.