Men are hunters
According to evolutionary psychology men used to be hunters and this left their brain in a state where they enjoy seeing weapons, fights and wars.
Most men love weapons
Men’s brains were wired in a way that most of them love weapons, horses, tanks and Armies.
They spark patriotism
Many war movies make people feel Patriotic. This makes them enjoy the movie with disregard of the killing that takes place.
Safe killing environment
The ideal scenario for a man is being able to watch a war going on while knowing that he is safe. The part inside the man that likes war gets satisfied while the moral part hardly gets offended as the man knows that it’s just a movie.
Testosterone give men aggressive tendencies. Action and war movies can allow some men to release some of that aggression in a safe way.
They love action not violence
Many of the people who watch War movies don’t like violence. Many of those people love the action, the adrenaline rush they get and the excitement they feel. (See Why Do Some People Like Horror Movies?)
Many war movies glorify noble concepts
Many war movies glorify certain noble concepts such as Heroism, Honor, Immortality, Sacrifice and courage.
They like History
Some war movie lovers enjoy war movies because they like to see some of the important historical events played in front of them.
Human emotions depend greatly on their perception of events. If someone was killed a bystander might not get disturbed if he believed that this person deserved to be killed. In most war movies killing is justified and viewed from a frame that makes it seem like a positive event.
Identifying with Heroes
When a person watches a movie he usually identifies with the Heroes. The person who watches a war movie might feel like he is a military commander who is saving his country.
Some people, usually men, enjoy feeling powerful and in control. When a person identities with a certain war Hero or a powerful army he feels more powerful and in control. This makes the movie more interesting to that person. (See Why are the Terminator movies so popular?)