You already answered your own question that you asked in para 1, in para 3. In spite of prophets who performed miracles, there are people who still dont believe in religion, which means that performing miracles has a very short term effect, probably limited to the direct observer or maybe to that and next generations. People who would come after couple of centuries will start questioning the authenticity of those accounts anyways.
Belief is a choice, and choice is the one that is important. Yes God could have made that belief part of our DNA, but then he already has Angels for that purpose.
Believing in a higher power is the most important. I doubt that God is going to punish you just because you use your left hand to drink water.