The circuit has to be low enough resistance to allow 5v to flow at 30 amps, and I imagine the resistance of human skin is just a bit higher than that.
A car battery is capable of putting-out hundreds of amps, but you can't feel anything when you touch the battery terminals....In order to get high current, you either need high voltage and/or low resistance. Since your body has high resistance, it requires higher-voltage before you can get enough current to feel anything.
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