There are two reasons why it is desirable to do so.
ASP.NET: Securing ViewState Efficiently
Even though ViewState is unreadable by the human eye, it is nothing but a base64 encoded string that easily decodes by a piece of software like this one.
Optimize ASP.NET Pages by Reducing the Size of the VIEWSTATE
One way to optimize the pages you develop with ASP.NET is to keep a check on the view state of the page and the controls that are added to the page.