The system the fight is in experiences time-dilation, where everything slows down to 10% speed or even less. However, a few effects create a positive feedback loop that makes the problem worse.
All the surrounding systems still run at full speed. You can travel large distances and still arrive soon enough to matter in the fight. You can also die, respawn in another system, rejoin the fight, and barely miss anything. The positions in the fight therefore move even slower than time-dilation since ships on both sides are replaced so quickly.
Large groups have a massive advantage over small groups, so alliances are very large and join various alliances-of-alliances. The playerbase is often organized into only 2-3 major coalitions. At some points in history, nearly all the alliances have joined the same coalition, which leads to a strange pax-Romana called the "blue donut" (referring to all the ownable outer-systems being "blue" or allied with each other).
Also, nearly every player in a large fight just follows simple orders. Orbit A and shoot B. There are just a few people calling the shots.
Fights sometimes end just because people are bored, need to sleep, or go to work.