I think it could be because there were miracles at the time the bible was written. Two people are sick, one touches the holy stone of Ptah and gets better, the other doesn't and dies. It's a miracle!
Miracles followed the same evolution as religion itself. The portable version of sacred stones, prayer, was developed and then, of course, there was a need for faith and lack of faith. Everyone can pray but not everyone gets the miracles, ah hah! They must have lacked faith!
Miracles haven't gone away since the bible was written, they've just evolved. There are people today who say that their illness was cured by a miracle from god and there are charlatans right now exploiting super-natural and superstitious beliefs.