Well, it was mainly sieges, but often sieges were more open than the ones we have. Basically they were often just an army camping near a city, preparing to assault it. And I agree that siegebattles are very effective, but I seek not effectiveness, I seek fun (not that you can't find those battles fun, nor me for that matter).
I absolutely love the fact that it took me three massive attempts to take Hatra from the Sassanids, and nother four to even lay siege to Ctesiphon. My armies simply had to retreat after suffering massive losses to the Sassanids fieldarmies (I won of course but it was costly each time).

Why sieges in large numbers are bad:
You eliminate the enemy army completely. Often in field battles the enemy army manages to reteat some important parts, and can thus easier reform for another battle.