I looked into this before, and i believe this is the reasoning. The element and cushion come as single unit. The passenger side cushion has the integrated scale sensor to determine how much weight is currently in the seat. For example, the passenger air bag is automatically disable if something 40lbs is in the seat, but remains activated if the weight on the seat is 0 or 170lbs.
I am fairly cheap, and would likely figure out if it's possible to retrofit a drivers side element onto the passengers cushion