Why don't modern trackball optical mice exist?

I've been wondering about this for awhile. Since logitech released their trackman and m570 models, it begs the question of "Why not just move the trackball to the bottom, so you can use it like a normal mouse?"

Couldn't there be a mouse that used a trackball and was crazy accurate, since it relied on a mechanical device instead of a sensor? Sure, it might slip and slide on your mousepad, but a grippy, ultra-textured mousepad could be engineered to accomodate for that. Then again, DPI may be an issue, but making the ball out of certain materials could accomodate for even tiny adjustments.

Or possibly, there could be just an advanced system of rollers on the ball itself, like mice from the 90s used, but it would be much more accurate. Just a thought, since I've used the m570 and I wish the damn thing would work like a normal mouse.