Because of the electrical and magnetic characteristics of a coil - but that would be best explained in the Electrical or Physics stack. Also, the capacitor and points / contacts are only designed for the small current flow and they have to operate at 2 * engine rpm (for a four stroke).
Usually 10 amps or less - the wires to the coil tend to be 14/010 same size as used for indicators etc.
See some detail here: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/ignition.html[How a coil works][1]