because they needed proof that an adaptation of a cartoon aimed at kids would work. ever wonder why DB was all of the sudden fast tracked into production last year after Transformers made boat loads of money? it's obvious, in my eyes, that Fox saw how well Transformers did in the box office and they decided to try to duplicate TFs results (lots of money) and rushed the movie into production so they could release it this Summer (get their money quick and soon). it's not like there was any news or rumors about the DB movie the 5 years that Fox sat on the rights. the fact that they're trying to GROUND Dragon Ball in a more realistic and modern world (instead of making it more fantastical) is another indication that they're trying to duplicate Transformer's success. they ARE just out to make a quick buck, otherwise they would have taken the time to make sure the movie is as good as it could be. instead they rushed the film into production.
whenever a film is RUSHED into production, it's because the main reason is to make a quick buck (X-Men: Last Stand).