Here's what I don't understand: why do cities keep growing? 

For most of history, cities were... the place you went to get free of your serfdom or your parents' expectations or your apprenticeship. They were hotbeds of plague and disease, with people living in impossibly close quarters, but then the alternatives weren't great either. At least in the cities, there were lots of opportunities right there.

After the Industrial Revolution, cities got bigger, dirtier, and richer. After antibiotics were invented and good sanitation practices became the norm, cities stopped being deathtraps.

And then dungeons started happening, and they became deathtraps again.

And people don't seem to have... noticed? cities keep getting bigger, and dungeons keep grabbing more and more people, and everyone is still going "oh I'd love to go live in NYC, but the housing is so expensive there". girl! the housing is expensive there and also you'll get nabbed by more dungeons and might die! or be permanently traumatized!

"oh I want to live three blocks away from fourteen ethnicities of restaurant" yeah and the dungeons want to live three blocks away from fourteen ethnicities of person, all of whom are tasty tasty treats. 

"I want to live near all my friends and their friends and their friends" we have the internet now! you can do work over the internet too! you can literally just live in the middle of nowhere and have the same social life! And, and this one is important, not get eaten by a dungeon three times a week.

"Dungeons do happen in rural areas too," I can hear you screaming. yep. more rarely, much more rarely, but they do. you do, in fact, have to take some other precautions, and make sure you have an alert set up in case of dungeon. but the most comprehensive precaution, the one that makes the most difference, is not living in a city. you should try it sometime.