This can only be a Steam issue, but I don't think I've ever seen it before!
Please try a **clean** reinstall of Steam and game:
- Remove the game from Steam (uninstall)
- Manually delete all these files and folders, if still there:
~/Library/Application Support/Steam/SteamApps/common/Cities Skylines/
~/Library/Application Support/Steam/userdata//255710/
~/Library/Application Support/Steam/SteamApps/workshop/content/255710/
~/Library/Application Support/Colossal Order/Cities Skylines/
(first move the /saves/ folder elsewhere to keep your save games)
- do a clean Steam reinstall
support.steampowered.com
- Install the game again and see if it runs now.
Good luck!