Several reasons. One the first "attempt" was so bad that I doubt Akira Toriyama will approve a live action DBZ movie anytime soon. Second, DBZ is a hard property to adapt. It's a sequel to a decidedly less popular series, and one that includes a lot of backstory and characters from that series. So you're essentially starting in the middle of an extremely long story.

Finally, the biggest reason is simply cost. A live action DBZ movie's CGI would put the budget around Avatar level.