Why isn’t there edible cardboard?

Cardboard by its nature of manufacture always has glass in it & metal. So is unsafe to eat. 

If cereal boxes were edible, no one would eat the dire contents

Even if it was made from edible materials the extensive processing to create cardboard would most likely leave it with very little or no nutritional value.

Its an interesting idea,  they have 'rice paper',  so.....

There are edible versions of everything that you mention, the problem with packaging is that you tend to use it to protect what's inside it!

*Rice is actually a more flexible base than cornstarch!

Diet popcorn. Some packing material is in fact made from corn products. Same difference.

They are working on it https://cen.acs.org/food/food-science/time-edible-...

Some packing peanuts actually Are edible-they just don't taste very good. But some cardboard can be recycled into paper for Newspapers.  

Cardboard is recycled and used again as is styrofoam and plastic' Some cardboard also has glue on it as well as staples along with ink and it wouldn't be sterile.