+1
What do you mean "have to be"? Nobody said it had to be. It's a stylistic decision, and people can make good or bad stylistic decisions. A better question is why you would expect different filmmakers to make the same kinds of films, just because they're about the same event. Similar to many other questions about why X and Y are not the same, when there's no particular reason to expect them be. People are different, and make different choices, is the answer to most of these questions.
BTW, it seems Greengrass made the better choice, given how well his film was received compared to the