Why has no movie version of I Am Legend dared to keep the original ending?

I'm going to have a go at answering all of your questions.

Firstly, I believe you have every adaptation listed above, with the exception of the trashy cash-in flick I am Omega, and perhaps, at a stretch, Romero's classic zombie films.

As far as I am aware, nobody has attempted an ending that echoes Matheson's original (and cerebral) conclusion to his novel, even though one might argue that the 2007 Will Smith vehicle didn't end with a classically 'happy ending' (although the ending they plumped for was far from satisfactory).

The simple reason for this is bums on seats. Movie making is a business, run by businessmen/women, and the dollar is the bottom line. Traditionally, the demographic that this kind of film is aimed at likes to see the (male) hero live to kick ass and kiss the girl - it's a brave (and rare) film that tries to break this mold. For the record, I think Darabont's take on The Mist is one of the greatest downers ever made.

As for Matheson's response to any of the films, I've dug around but haven't unearthed anything. Perhaps another user can dig a little deeper.