"Because the current wants to get back to the transformer on the street not to the earth."
This may have already been said or implied, but the voltage to ground of an ungrounded xformer secondary is indeterminate.
Because of interwinding parasitic capacitance, the secondary voltage may float up to the primary voltage, or it may not because winding capacitance to ground may overwhelm the other capacitances.
Grounding the center tap somewhere makes the secondary voltage with respect to ground predictable and independent of stray capacitance; that is, a max of 150vrms.
Mr. 75, there is more to that article than meets the eye. It may save lives.