Haven't played Cities: Skylines for ages, so I re-installed it on Steam, after some updates there's this new Paradox Launcher which serves no purpose but wasting time. So I decided to remove it, googled some guide and unfortunately it contains too many steps and some error occured while following it. Here's the simpler & correct step of what I did instead.
After the game is downloaded, open the Steam library, find the game, right click, choose
Next for the
Launch Options, type
cmd /c start cities.exe %command%
/Users/$USER/Library/Application\ Support/Steam/steamapps/common/Cities_Skylines/Cities.app/Contents/MacOS/Cities %command%
Make sure you replace
$USER with your username
Then the game works like good o' times, straight to the menu.