Stormtroopers act like professionals? What fictional universe do you inhabit? Certainly not the "Star Wars" universe. Let's go right back to the very first episode. As Obi-Wan remarks, "The Force can have a strong influence on the weak-minded."
Stormtroopers are naturally weak-minded, so of course they act unprofessionally, even without nudging from the Force. As the saying goes, "I can do stupid all on my own." The first episode demonstrates that they can't even shoot straight, so there's no particular reason to think they can anything else right - at least, not if such failure advances the plot.
EDIT - I think I should expand on my last point.
First, we need to step out of canon. The first Star Wars was clearly intended as essentially a comic book come to life. Harrison Ford's classic, "George! You can type this shit, but you sure can't say it!" does rather describe the dialog. And George Lucas wanted what he wanted from the visuals - and he got it. This was fine until folks started taking it seriously and attempting to incorporate the movie into a "real" (whatever that means) world.
So let's take the Stormtroopers at face value. How are they as soldiers?
1) They wear armor which does not seem to actually work. I've not watched the original in quite a while, but I do not recall ever seeing a stormtrooper's armor deflecting a shot. If they get hit, they go down. Also, just as a nitpick, you might notice that nobody ever gets hit in the arms, legs, or head. What are the odds?
2) They carry "laser" weapons with optical sights - and they never use the sights. It's all shoot from the hip, and with terrible accuracy. Green beams flying all over the place and the occasional casualty. Any US Marine today would be ashamed to do it that way. The only reason they don't get slaughtered is that the rebels are just as incompetent.
3) While #2 might possibly be excused for close-in shootouts, where close range and stress can cause people to do all sorts of funny stuff, the same does not apply, for instance, to the escape of the Millennial Falcon. Here, the troopers are relatively far from the ship and only Han Solo is providing covering fire. He should have been chopped about 3 seconds after the shooting started.
(Stepping out of canon for a moment, all of this is necessary to advance the plot, while looking good at the same time. Shootouts with laser beams all over the place, heroic covering fire, etc. Pity it doesn't make any sense.)
So, what have we got? Soldiers who are burdened by inappropriate equipment, and who are incapable of hitting exposed targets at close range and/or completely exposed. In other words, amateurs. And these are supposed to be elite troops?
Well, yes. Yes they are. And since they are supposed to be elite, they ARE elite - the movies and novelizations say they are.
There is an old saying about movies: "Show, don't tell." And what the movies show are a bunch of wankers. Apparently the rest of the Star Wars universe is made up of even worse troops, so they are elite wankers, but wankers nonetheless.