Maybe because signing a new deal is better for your country and your people?
Like what's the worst case scenario that happens to iran if they just signed a new deal? How would it have been any worse than the current situation?
The deal doesn't even require a high level of trust - worst case they just go back to the previous situation. Making a deal only has an upside, even without trust.
Just like employment at will - and yet having a job is better than no job in most cases.
Israel would be deeply impacted by the results of the negotiations, so is this surprising or unexpected? Any nation including the US would most likely do the same in a similar scenario over what is considered to be almost existential negotiations.
1. Do what they are doing now.
2. Sign a new deal and see where it goes, worst case they go back to #1.
The downside of signing a new deal is practically non-existent.