I have been driving for 30 years and can count on one hand the number of times I have parallel parked—and would have too many fingers left to give a New York wave. I vaguely remember how to do it, and will flub it the next time I have to. Just hopefully not that bad.
I agree that it's infrequently used. Especially out of urban areas. However practicing and being tested for it improves close quarters driving skill and ability to maneuver.
When and how soon to cut the wheels, and how much. When to straighten out. How to judge closeness to the vehicles in front of you and behind.
If you want to see the lack of this type of maneuvering skill, go to a boat launch ramp. It's hilarious to watch some of these these people. They're turning their wheels the wrong way, and get everything so crossed up they're lucky if they have the room to pull forward to straighten out.
Most can't even back up a jet ski trailer straight for 20 feet without jackknifing it. You would think they would go to a parking lot early in the morning and practice. But most try to learn at the ramp. It's almost become a spectator sport in itself.