'Cause it's in third person. The novel is in first person.
Of course, movies are mostly in third person. I supposed I could imagine a first person movie--lots of subjective angle POV shots--but it would be unsufferably avant-garde. There have been movies with scenes done that way, of course, lots of them. The first one I can remember was John Carpenter's Halloween, which had long scenes from the killer's POV. This was considered spine-tinglely transgressive, and I recall lots of learned commentary about how we were now implicated in the killings or more inured to violence, or whatever. I remember not caring--I was a teenager, and I thought I'd just seen the coolest, scariest horror film ever.
But most films, the camera is neutral, objective, recording the action as a kind of omnipotent, often distant arbiter. The Hunger Games film does something interesting--the early scenes, with Katniss hunting in District Twelve, are more intimate, more hand-held camera--what my wife calls shaky cam. The objective observer is a close friend, perhaps, or like maybe a bird. But when they get to the Capitol, the camera's further off from the action, recording it sardonically, so we can get to see all the production design for that oh-so-sophisticated Big City. Then, when the Hunger Games begin, we go back to shaky cam. But there aren't many shots--maybe not any--from Katniss' POV.
The entire novel is told from Katniss' POV, and it's effective. She's an agreeable, tough, brave young woman, and we don't mind living in her head. My favorite novel ever is Huck Finn, and that's also first person, from the point of view of a very bright and observant, but not remotely sophisticated young person. When the Hunger Games begin, the novel gives us a sense of just why they're called that, how it's not just about killing, it's also about survival at its most basic. Jennifer Lawrence is brilliant as Katniss--she conveys Katniss' courage and loyalty and essential decency, but the movie, because it's a movie and shorter than a novel, doesn't do as well showing Katniss' fight with hunger and cold and injury and illness.
But the novel is about a teenaged girl, for a readership of teens, and quite a bit of it is about 'I like this boy a lot, but I'm not sure he likes me the same way, and this other boy likes me a lot, but I'm not sure I like him the same way' kinds of thoughts and feelings. None of which I cared about, pretty much at all. I'm not who the novel is meant for.
What the movie does instead is get into all sorts of things I was really interested in that the novel, because it's first person, couldn't even touch. Like politics. President Snow is played by Donald Sutherland, and is such a terrifically compelling presence in the film that even with less than three minutes screen time (my estimate), he kind of dominates. I love the scene between him and the Game Designer (himself a terrifically interesting character who doesn't appear in the novel at all), where Snow says 'have you ever met 'the underdog?" Chilling stuff. The novel gives us one brief glimpse of Snow, exactly the kind of sociopathic autocrat who you can see rising to the top of a society like Panem--a sort of bearded Putin--but the movie fleshes out the character in wonderful ways. And the Senecan suicide of the Game Designer: that scene was superb. The actual Seneca, the Roman Senator, philosopher, and playwright Seneca, accused of treason, was told he would be arrested within one day. So he took a warm bath, and opened his veins, chatting calmly with his wife and family while his life drained away. The Game Designer is murdered with similar sophistication and also, just a touch of murderous wit. The character's name? Seneca Crane--Wes Bentley in a terrifically intricate beard.
The movie follows the novel really closely. What it doesn't do is what the novel does easily--get inside Katniss' head. What it does instead is dissect the Games, and the society that produced them. And I found that much more interesting. I liked the novel fine. I liked the movie a lot.