What possible reason could there be to have champions lose non-title matches every single week on RAW? Wouldn't having your champions seem special & nearly unbeatable be a good thing? 

Because that's literally the only way they know how to build to title matches.  And if stuff built logically it'd be fine, like if Baron Corbin beat Seth Rollins to set him up as a challenger at WWE Endless Breadsticks, but it's not built that way.  It's just everyone has to be kept on the same level and beaten down until ratings drop below the levels anyone thought was possible.