Why do the inserts in pasta pots not go all the way to the bottom?
You might have given the answer in your question.  If the device is to be used as a steamer as well, then there has to be enough room at the bottom of the pot for steaming.  How much is enough?  There are two things to consider:


The bottom section should hold enough liquid that there won't be concerns about boiling the pot dry for anything you might want to steam.  Longer steam items might include mussels, lobster, or tamales.
There should be some 'air margin' for foods like tamales or broccoli, so that the boil splatter doesn't render the food soggy.


Based on FuzzyChef's comment, it sounds like there are tools that suit your needs better.  Perhaps it's time to put your steamer on Craigslist? :)