Why are end cities not generated in survival, though in creative they are?

End Cities don't strictly follow the seed, they're dependent on rng (random number generator). Similarly to how you get a random amount of diamonds between 1 and 4 with fortune 3, or how when you get a mob spawn somewhere 'random'. Each end city has a pre determined position to generate along with a probability to generate. The output of RNG will decide if the end city should generate which is done upon loading chunks hence why end cities may not appear even when copying a world and going to the same location in both. I'd suggest using ADMIST if you want to find end cities and go from location to location until you find one - doing this in creative before hand is a waste of time.