Whether you are correct or not, you see, *I* don't get plenty of swimming in 5 or 6 feet of water. I like to swim with my children. My five year-old is also a good swimmer and although 5 to 6 feet is adequate for him (now), even at 5'4" it's rather shallow for me to do a descent tuck dive. I'd like to teach him someday, but I'm not cracking my head on the bottom of the pool to do so.
My five year-old loves diving to the bottom after various objects. Our three foot hot tub is pathetically shallow. Two more feet makes it a bit more exciting for now, but it won't be long before he's capable and interested in going deeper.
I agree 5 or 6 feet may be adequate, but who in the world wants to live with adequate when superior is possible?