I'm just trying to learn: If the GFCI won't trip when you drop a hair dryer into the tub, then why have GFCI's ?
Electricity is used by many but many don't understand it.
For current to flow there has to be a complete path. Birds can sit on a power line and not get shocked because there is no path for current to flow.
A fiberglass bathtub plumbed with plastic pipe is a very poor conductor and therefore doesn't provide a return path for current to follow.
A ground fault receptacle monitors the current in the two straight pins. If it is equal then there isn't a fault. If it is different then current must be flowing through some other path other than the hot and neutral.
By dropping a hair dryer into a bathtub plumbed in plastic there is no
"third conductor" and all the current in hot pin can only return through the neutral pin and so there is no fault.
Plastic plumbing has removed many of the ground paths that used to be in a house.