Power
Imagine you're an alarm clock, and at the appointed time you shout, "Time to wake up time to wake up time to wake up." Although you're technically correct, no one has to listen to you, and they might even throw you out the door and into the hallway. And so it is with city planners (except for the part about being thrown into the hallway. That almost never happens.). Notice the job title is "city planner" and not "city plan decider."
Ultimately, the decision to use or reject your long-range plans is up to someone else, such as the town manager or planning committee. Those who go into this career with dreams of shaping their community are in for a rude awakening, to keep with the alarm clock theme here.
You may have an awesome plan for zipline transportation from the Senior Center to the mall food court, backed up by all the facts and figures and sheer gut instinct. But all it takes is for one committee member to say, "Ehhh, I'm just not feeling it," and it's back to the drawing board.
Or you could persist, and after much badgering and perhaps a little political intrigue, you get your plan approved. The zipline is a huge hit, and it energizes the whole community. Now you have a bit of power that you can use to sway things your way when trying to get approval for future ideas.
Of course, the opposite could happen, and the Great Zipline Disaster relegates you to rubberstamping zoning permits until you can find a job in a city far, far away.