It’s the current that kills you, not the voltage !
The problem with this saying is that it misleads people into thinking that high-current power sources are more dangerous than high-voltage power sources. Most power sources are voltage sources, not current sources. This means they output a constant voltage, and the current in the circuit depends on the resistance of the load (the human body, in this case). This is true for power lines, batteries, etc. Most people don’t understand that the current listed on a power supply is just a maximum rating, and won’t actually go through their body if they touch it.
If you connect a 1 kΩ resistor across a 12 V supply, the same 12 mA of current will flow whether the supply says 100 mA or 100 A. The amperage rating of a power supply just tells the current it could source, if connected to a small enough resistance. It doesn’t force that amount of current through anything it touches, or it would be constantly arcing through the air