I Am Legend: Why Can't Matheson's Masterpiece be Done Justice on Film?

To answer the question in the link here, the reason is likely, in any era, movie makers think they need to have a spicier product. The book (SPOILERS) is more cerebral, even the "climax" is a little anti-climactic as Neville getting captured happens with a huge fight of any kind. All there is is the "new species" killing off the dead vampires and capturing Neville, and to soon dispatch him.

I think I'D like a faithfully adapted movie, I just don't think the people putting up the tens (or hundreds)of millions to make a theater-level movie will want it to be faithful. They're going to want action, and likely explosions, etc, and a reasonably happy ending. Audiences aren't going to line up and return the investment that has a movie ending with the "protagonist" dead and the "freaks" ascending.

As for the embedded reviews, the author loses credibility when he praises the latest attempt. The monsters are pathetic, the CGI is terrible, the acting is worse than Heston's, and it is also far, far from the book. I have a soft spot for the Omega Man as it is the first that I saw, and it scared the used-food out of me as a seven year old, and I looked forward to scaring myself over again when it was re-run. But the Last Man On Earth is probably the most faithful overall, and it is much sadder. And it is the only version which has the mutated ascending to prominence.