Usyk has great defense but he isn't labeled as a "defensive fighter"?

He's an aggressive boxer/counter-puncher.

Always forcing the action, throwing in combination while being defensively responsible.

The 12th round of the Michel Hunter fight is a good example.

 

He is not a purely defensive fighter.

At 6'2 and a half, if he decided to be a backfoot defensive fighter like Bivol, he would not have beaten AJ or Fury. He can't box super defensive vs much taller man.

Bivol is a great example of a predominantly defense first fighter. Usyk is cautious on offense and only attacks when he is certain he can land without being countered. But he is an aggressive front foot fighter. Just extremely calculated aggression with strong defensive responsibility.

 

His defense and offense are both at an ATG level

 

Usyk at his core his roots is a pressure fighter and it's how he managed to beat Fury

If Usyk went defensive and survival mode after them tough middle rounds to let Fury breathe then Fury would have walked it but he remained true to his gameplan and forged his way through fire to the other side.