Why don’t more people who are righties bat lefty, since your power hand when you bat left is your right hand?

 I see 2 reasons -

1. In cricket, especially in early coaching days, importance is given to free movement of your front foot. Back foot is supposed to be stable. Hence, whether you are left-footed or right-footed also matters in addition to your handedness. In general, right-handed are right-footed. Of course, there can be exceptions.
   I am left-handed and left-footed. But I bat with RHB stance. So, I tend to move my left-foot freely. It helps in having left-foot as front foot to freely move across for playing drives (esp. cover drive) and flicks/glances.
2. I personally think the bottom hand is the power hand in cricket - opposite of what you mentioned in your question. Your top hand acts as pivot and the bottom hand to generate power - especially for pulls, cuts, lofted shots.
   Again, in my personal experience, being left-handed and playing RHB stance, my top (left) wrist tends to move quickly, thereby shifting the bat to leg-side resulting in frequent leading edges. And, I find it difficult to drive on off-side.