Why did the war last so long?

Answer 1

Because it had become a stalemate, and Grant settled on a policy of attrition - ending the system of prisoner-exchange, knowing that the Confederates could not replace their battle-losses.

The war ended when Lee simply ran out of manpower.

Answer 2

The war lasted so long due to factors such as entrenched political ideologies, economic interests, conflicting territorial claims, and the involvement of multiple powerful nations. Additionally, military stalemates, advancements in technology, and the scale of the conflict all contributed to prolonging the duration of the war.