Extradition treaties can be tricky. They are not based on whether 2 countries accept all provisions of the other's justice system.
Each treaty sets provisions under which an extradition is possible. Extradition would need to be formally requested by the Italian government. A US prosecutor would be assigned to review the evidence, a judge would likely need to agree that the evidence is sufficient to warrant a trial and then an extradition hearing would begin. That can take years.
With the Knox case - part of the treaty states that: "Extradition shall not be granted when the person sought has been convicted, acquitted or pardoned, or has served the sentence imposed, by the Requested Party for the same acts for which extradition is requested."
In this case there was already a full trial, a recorded conviction, a sentence and partial serving of that sentence - then verdict was overturned.
Knox also didn't return to the US as a fugitive, she was released by the Italian Justice System - post trial.
These are very unusual circumstances for extradition.