Who would have believed anybody who talked? Miracles are ultimately useless in conversion because 1) If it didn't happen on TV it didn't happen, and 2) If it DID happen on TV it must have been faked. See Exodus. People take the miracles then always demand more. No, faith is built on something other than miracles.
Generally true, except for these ongoing miraculous events (except for Fatima, which is otherwise well-documented):
Eucharistic Miracle of Lanciano
The Tilma of Guadalupe
Incorrupt bodies of the saints
Fatima
Shroud of Turin
Sudarium of Oviedo