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The only reason Luke, Leia, Han and Chewbacca were able to escape the Death Star is because Stormtroopers left their stations and ran like a kid to watch lightsaber duel between Vader and Kenobi.

Why didn't stormtroopers act like professionals? If they were so much interested, they could watch hologram or video of a lightsaber duel later.

Though whether Darth Vader actually held rank in the Imperial military was unclear, he is still at the very least a VIP aboard the Death Star. The stormtroopers choosing to provide such an important figure with backup when he is engaged in combat with an unknown foe is perfectly reasonable.

In addition, the stormtroopers aboard the Death Star were specifically instructed to allow the Millennium Falcon to escape after the tracking device was installed so that they could be tracked to the location of the rebel base. Leaving their posts to provide aid to Darth Vader is a perfect excuse to do so.

According to the film's original novelisation, the guards were originally attracted to observe the fight, then moved into position to help Vader.

Illuminated by the flare from contacting energy fields, Ben Kenobi and Darth Vader were backing toward the bay. The fight attracted the attention of others beside the Senator. Every one of the guards moved in for a better view of the Olympian conflict.

“Now’s our chance,” Solo observed, starting forward.

All seven of the troopers guarding the ship broke and rushed toward the combatants, going to the Dark Lord’s aid. Threepio barely ducked aside as they ran past him. Turning back into the alcove, he yelled to his companion.

_{^{A New Hope: Official Novelisation}}

This tallies nicely with the film's script

Vader and Ben Kenobi continue their powerful duel. As they hit their lightsabers together, lightning flashes on impact. Troopers look on in interest as the old Jedi and Dark Lord of The Sith fight. Suddenly Luke spots the battle from his group's vantage point.

...

Threepio ducks out of sight as the seven stormtroopers who were
guarding the starship rush past them heading towards Ben and The Sith
Knight. He pulls on Artoo.

...

They start for the Millennium Falcon. Ben sees the troops charging toward him and realizes that he is trapped. Vader takes advantage of Ben's momentary distraction and brings his mighty lightsaber down on the old man. Ben manages to deflect the blow and swiftly turns around.

_{A New Hope: Script}

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.