Why have static targets for tributes while demoing their skills?

There is no specific In-Universe answer in either the book or the movie for the static target. You are left with your own imagination/interpretation as to why.

Not even a reasonable supposition?

Well, Katniss is not an exceptionally favored Tribute. Aside from her volunteering 'stunt' at the Reaping, she's still a District 12 tribute. Not exceptionally strong, charismatic, intelligent, or ruthless. She's average. She's young. The odds are not in her favor, not compared to the Careers, specially trained 18 year old killing machines. Katniss doesn't play the politic game of the Hunger Games yet.

Taking that into account, and how the training sessions don't seem to be televised (unlike the interviews with Caesar), the demo were staged by the game makers, for the senior game makers. First, the game makers are not even remotely interested in Katniss when she first walks in, because of the above. Not favored to win, not charismatic/politician. She's done nothing to draw in their attention. Two, the game makers underestimate her ability with the bow. Three, Katniss is the last of 24 Tributes:

After about fifteen minutes [After Peeta went in], they call my name. I smooth my hair, set my shoulders back, and walk into the gymnasium. Instantly, I know I’m in trouble. They’ve been here too long, the Gamemakers. Sat through twenty-three other demonstrations. Had too much to wine, most of them. Want more than anything to go home.

So they don't bother making an elaborate demo for her. Why bother when she's not likely to win or make for good entertainment. It's a waste of effort, time and money. This signifies a complete lack of interest in Katniss. Katniss is boring to them, so her demo is boring. (And because her Demo is boring, Katniss will continue to be boring, a vicious cycle right?)

Her righteous anger and the Pig Stunt changes that dramatically. Katniss clues in on how she needs to play the game, by playing the people. By acting. The literary answer as you mentioned, to shock both the reader and the characters. Her stunt causes everyone to look at her as a viable competitor for the first time. The following year, they put up a force field. And the demos are less boring, as all the Tributes are proven Victors.

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That said, the scene is a bit different in the book, short but exciting:

I choose a bow, string it, and sling the matching quiver of arrows over my shoulder. There’s a shooting range, but it’s much too limited. Standard bull's-eyes and human silhouettes. I walk to the center of the gymnasium and pick my first target. The dummy used for knife practice. Even as I pull back on the bow I know something is wrong. The string’s tighter than the one I use at home. The arrow’s more rigid. I miss the dummy by a couple of inches and lose what little attention I had been commanding. For a moment, I’m humiliated, then I head back to the bull’s-eye. I shoot again and again until I get the feel of these new weapons.

Back in the center of the gymnasium, I take my initial position and skewer the dummy right through the heart. Then I sever the rope that holds the sandbag for boxing, and the bag splits open as it slams to the ground. Without pausing, I shoulder-roll forward, come up on one knee, and send an arrow into one of the hanging lights high above the gymnasium floor. A shower of sparks bursts from the fixture.

It’s excellent shooting. I turn to the Gamemakers. A few are nodding approval, but the majority of them are fixated on a roast pig that has just arrived at their banquet table.

In the movie, she has their complete attention, but her single shot was off target, losing the Gamemakers attention completely before her second perfect shot. And the pig was already there. Compare:

Also missing from the movie? One of the Gamemakers falling back into a bowl of punch from shock of Katniss shooting the Arrow. That Gamemaker? Plutarch Heavensbee, as we learned in Catching fire.