Why do I2C lines use open drain driver instead of tri-state drivers?
...as long as the enable signals for the tri-states are mutually exclusive...


The trick is how to do this without adding another wire, or multiple wires to tell each peripheral when it is allowed to drive the bus.

The main advantage of I2C is that it only uses two wires, and two pins on each chip connected to the bus. 

If you're willing to trade pins for speed, consider using SPI, which can generally achieve higher speed than I2C, but needs 3 or 4 pins per device.