You are forgetting that Zanarkand was destroyed and that Jecht and Tidus are from the dreamworld of the Fayth. Jecht was pulled out of the dreamworld by the fayth or Sin, not to sure which one but when Jecht touched Sin he became real, and since he became Sin he helped Auron to go to Zanarkand(dream version to look after Tidus. Sin(Jecht) decided to pull his son out of the dreamworld in hopes of defeating him(Sin) and putting an end to it all, and helping the Fayth to not have to dream anymore.