Because every system can be exploitable, especially one that is as strict is this. Tuchel is not known for sticking with a certain formation. What he is doing, or at least I hope he is doing, is building the foundation from the back. Doing the temporary fix to get the team performing again.
The great thing about this temporary fix, is that we are still playing progressive football with the possession game Tuchel wants. So we are still on the right path.