Because treaties like JCPOA aren't based on trust, they're based on verification. Trust, in the sense you're describing it (the way a person might trust another based on their past behaviour) doesn't really exist between nation states.
And Iran is very incentivized to develop nuclear weapons. North Korea's bargaining position wrt to the U.S. improved immensely once they demonstrate a nuclear weapon; Pakistan and India have both enjoyed much less superpower pressure because they also have nukes. And more immediately, Israel has nukes and are Iran's primary regional rival. Israel has been a dominant regional entity largely because of it's military superiority over everyone else, which includes its nukes. Iran obtaining nukes is a potent counterweight to Israel's perceived freedom of military action.
And Iran is very incentivized to develop nuclear weapons. North Korea's bargaining position wrt to the U.S. improved immensely once they demonstrate a nuclear weapon; Pakistan and India have both enjoyed much less superpower pressure because they also have nukes. And more immediately, Israel has nukes and are Iran's primary regional rival. Israel has been a dominant regional entity largely because of it's military superiority over everyone else, which includes its nukes. Iran obtaining nukes is a potent counterweight to Israel's perceived freedom of military action.