Prometheus

[Cranky Old Man]

There usually not enough budget for extra people who won't bother to help out for free, AKA a scientist. They have things to spend better time on. That's most of the logic right there. With a movie like Prometheus, all the budget went into costume, prop, makeup and special effects on top of all the sound designers, modelers, texturers, etc. A lead animator in that industry will get paid roughly 800-1k for a second of FINISIHED animation, a lot of the other people will be paid roughly anywhere between 30k (new hire, not as much experience) to 60k for a salary. Then you have to pay for all the facilities, programs, food, etc. Actors usually get paid way more than they should.

So if you have 130 million, you'll most likely blow through most of it in the first couple months of producing a film. Since most movies are a year process of filming, in about 6 months, you'll be down at least 90 million. That's just to pay all the people you already have. Then add in all the people you haven't paid yet, who haven't worked yet. Films are expensive. They are hard work. Hiring randoms is a waste of money in the directors and producers eyes. You truly have to factor in every bit of the production before you can say "oh they should've just hired some scientists who would have needed to be paid at least a hundred thousand because you're wasting their time and they might talk about the movie and leak information if not bribed properly" (everyone who works for the film has to sign a contract, a scientist offering ideas is not someone who is working on the film, they are a paid extra, in film terms).

So basically what you're saying is that they blew their budget. That's a rookie accountant mistake, and usually you can always loan money from the movie company if you really need it. Also, while scientists are expensive, nerds aren't. If you replaced Lindelof with a nerd, I bet you'd have a better and cheaper movie.
Also, the movie makers need to get their priorities straight: Foremost, a movie (Well, most movies.) is supposed to make sense. That's all it really needs to do. Everything else is secondary. There are people who have built careers on doing nothing but complain about movie plots that doesn't make sense. You don't see anybody building careers over complaining about lighting, sound quality or the wardrobe, because that's not as important as the plot. (CGI has a niche too, and the quality of the acting, so those come second to the story making sense.) A plot doesn't cost money. There's a surplus of plots and writers in the world, and most of them can write better stuff than this plot.