That is because of the inexperience of the centres (e.g. Kata) and the incompetence of the coach. Kata will get better over time.
Incompetence of coaches is definitely a factor - but not so much inexperience - they flat out can't defend.... Laumape was the only one that could, and he's gone
Even with our perhaps our 'best' lineup:
1 Tomkins
2. Anyone
3. Kata
4. Hurrell
5. Vatuvei
6. Townsend
7 Johnson
Every one of them is a poor to abysmal defender - ironically Johnson probably one of the better ones
Next year clearly there is a huge upgrade at fullback, and presumably Leuleuai or Lolohea will be at 6 - Thomas is a solid defender, Tui was exploited by Penrith
I worry for RTS tbh, going from a well-drilled professional unit where he can excel, to our rabble