"In several videos and photos you see people getting round[ed] up... Why does nobody try to run away?"
It's not that nobody ran away, it's that nobody ever took a video of people running away.
People ran away all the time. They ran, they hid, they begged and they fought. The individuals who survived were for the most part of an age and background that were capable of running and of hiding. But when people got caught, turned in or trapped, a very human response at the end of all things is to do pretty much nothing.
Friedrich Jeckeln was an Obergruppenführer in the SS, who was highly decorated for his role in standardising the procedure of pit shootings. One of the things that he noted (although it was not put into practice in the picture above) is that there were fewer attempts at flight if you made people undress before approaching the pit. His implementation of this idea also speaks to the fact that, fully dressed, many people did attempt to flee at the last minute.
Such flights were all the more common in circumstances in which the shootings were conducted in an erratic manner. Where they were organised and streamlined, resistance was minimal. Again, this is a very human (lack of) response.