Tubers are used because the type we eat are tetraploids. Tetraploids (aka 4 copies of every chromosome) produce very little seeds. Generally less than 1/10th what a diploid version will. In potatoes it can be a low as 1/10,000th.

Using tubers transmits all sorts of nasty diseases from one crop to the next. Seeds do too but not as much.

Diploids are not used commercially because they produce smaller tubers and longer vines.