If humans were so advanced to have city planning at that point, how do we only have mobile phones by now?

There is a reason we name eras after materials - the bronze age, iron age, etc. Currently we're living in the silicon age.

Progress in fundamental materials science tends to unlock whole new technology paradigms.

You can do city planning with sod and stone. Mobile phones, on the other hand, require a nearly incomprehensible level of materials innovation. It is everything from the battery to conductive touch screen glass to plastic casing to silicon microchip... Not to mention all the science of satellites and rockets and radio waves that make them useful...

By the way, the show "Connections" by James Burke is brilliant. A must-watch for any tech curious nerd.

Because city planning doesn’t require the same technological advancements that a cell phone does?

Human sophistication and intelligence is not the same a technological advancements.

Maybe they did but became enlightened and destroyed their phones after versions of Facebook and Twitter cause their civilization to collapse?