why doesn't jocasta trust prophecies?

Jocasta thinks that the prophecies are pretty much bunk. She does not believe in the truth of oracles or the "seer". One can interpret that, by extension, she doesn't put much stock in the existence of the Gods.

Jocasta and king Laius believed in Oracles' prophesies once, when they were told about their son's horrible doings in future, who was just born at that time. By the time they got rid of upcoming disasterous event so Jocasta thought it is all over and prophesy didn't come true. Now Jocasta has stuck to the belief that prophesies don't come true to one's life.