Because after they saw signs: Moses Burning bush, parting the sea, healing the sick, blind to see, raising the dead, in the end they FORGET what they saw and continued in their disobedience, living in their sin like nothing ever happened and those signs never rooted in a changed heart.
People don't need to have signs so they will be able to then save themselves. People need to be saved because they are incapable of saving themselves even when they have had the signs to see for themselves with their own two eyes.
EDIT: BTW you can read it in your NT. They asked why and they were told why. "I'm done with signs - you don't pay attention or listen and still simply do what you want anyway" (quote paraphrased)