Because from a distance it actually doesn't seem like so much work. The assets get crazy expensive, but they already had that covered. 3D collision detection an

Because from a distance it actually doesn't seem like so much work. The assets get crazy expensive, but they already had that covered. 3D collision detection and resolving is a solved problem, you can just get off the shelf components (for free), the real problem isn't gettin the game to work, the problem is in the polish. Jumping over fences, AIs chasing over 3d terrains, making sure people don't get stuck in random stuff. Everything needs weird edge-case stuff that's not in a generic physics engine. Perhaps they should've first made a 3d game with 2d physics, those games look great and the overhead is much more manageable. But hindsight is 20/20 and from what I've gathered they actually got really close to a great game.