I have already an account whose base is level 17. Just wondering why I have to build another base from..

Not sure what you mean by having to build another base? If you start a new game then you will begin again with a base in the latest state to be created. However you can move the base to any state as long as you do it before you reach level 6. Many people do this to have a farm account or simply play 2 bases at once.

If you have created a new game by mistake or on a new device and want to get back to your original game then just play until you reach a point where you can access the settings menu. Then choose the account option and log back into your original game with the details that you bound it to.