A true point with many examples. The Lord of the Rings, as written, would also make for a very boring movie (all that tedious elvish and whatnot).
But the screen "adapter" of Starship Troopers didn't even try. He just cobbled together a hackneyed collection of his own ideas and prejudices, loosely glued on a stripped down summary version of the book's plot, and dishonestly tried to pass it off with the Starship Troopers title. And then called it "satire" when fans weren't taken in by the butchery. Note that they only stared calling the movie a "satire" after they found they'd pissed off a lot of R.A.H. fans with their crappy little movie.
If hollywood really wanted to do a satire of Starship Troopers, they'd have given the project to someone like Sam Raimi. He'd at least have the honesty to call his film something other than Starship Troopers. That, or why not make a movie of Bill the Galactic Hero if they're so desperate to make a movie satire of Heinlein? Bill would have made an excellent film with only minimal "adaptation" (especially the fuse room scene). I have no problem with honest satire. What I can't stand is a dishonest slash job.
Heinlein would be a laughing stock if translated straight from book to screen. He was very much of a particular era, i.e. Farnham's Freehold.
Some of his plots are definately dated. But the themes and ideas he presents are pretty damn timeless and universal. Many of his short stories (i.e. Tunnel in the Sky) would make for damn fine movies.
But hollywood simply can't be bothered to read, understand, or even stick to works of science fiction except where it suits their ideologies (such as when the author is suitably socialist). Just look at the steaming pile they tried to pass off as I, Robot.
You give Hollywood both too much and too little credit. Although the financing of movies is pretty much down to five companies (actually virtually ALL popular culture is created by those five companies) beneath the surface it's chaotic. Writers, directors, actors etc. fighting for projects, money, etc. Scripts being re-written at the las minute. Actors and directors being replace. For instance, the movie Pretty Woman was originally written as a very dark social commentary and then changed as they were shooting it to romantic comedy. So who knows how Starship Troopers, by that most American of Scifi writers, started out or how it was seen by a dutch guy who directed both action adventure and social commentary.
Who even knows how they edited it or re-cut it once it was shot? Body Heat, for instance, was re-cut (brilliantly) at the last minute with something like 20 minutes edited out of it. If you look closely, you can see conversations between two characters taking place while they are in different locations.
That said, there is no "Hollywood" as you think of it. No monolithic organization with a single vision -- at least not a vision that extends beyond keeping their jobs and making money. I've met a bunch of Hollywood producers, directors, etc. over the years through odd twists of fate and I can state without hesitation that they are the most frightened people you would ever hope to meet. They are in a highly competitive field that offers big money, but only if you "guess" right almost every time. It's pretty much like having a high paying job where the boss tells you to go into a casino and make three very large bets. If you bet right, then you get to keep your job. If you guess wrong, then you're fired.