Why pro-e is parametric and not AutoCAD?

Parametric is something which derived dimension from 3D model. 

Both software are parametric

Pro-E otherwise know as  CREO which is parametric software where 3D model is created from detailing is created.

Autocad is also a parametric software but not to that extend of Pro-e OR CREO act as parametric. You can find a lot but below is a path from one example 

http://www.engineering.com/DesignSoftware/DesignSoftwareArticles/ArticleID/11474/Introduction-to-Parametric-Drawing-in-AutoCAD.aspx