Probably because the bigger the special effects budget, the more likely they are to scrimp on a budget for a fully fledged and logical script.
Just look at the Transformers movies. I believe there was rumors that Transformers 2 originally had a promising script, but then someone was brought on to chop it up and make it cheaper. Presumably cheaper to film.
Regards,
SB
Nope.
What happened in this:
The writer's strike happens. Bay is left with pre-production on a movie and the writers he's working with(Which you'll recognize from Star Trek. They wrote that too, and produced it) are currently, well, striking. So, he gets a brilliant idea in his head.
"I'll write the first draft!"
And all they could do after the strike is over is try to patch up what must have been an abortion in the form of a script.
=\ that's all there is to it, really.