This means that the animation option in your game is turned off.

Open the game's settings (the round yellow cog button at the bottom of your screen), then you’ll see a button with a picture of a person. You can turn the animation on and off by clicking on it. It will be Yellow when it is turned on.

If your graphic settings are set to low, you won't be able to turn on the animation.

Also please note: in order for people to walk around the city, there should be sidewalks or footpaths in the city. They can be purchased in the game store, in the "Roads and Decorations" section - the "Fences and Roads" tab.