Because they have more underlying factors such as (straight) brown hair or (curly) blonde hair from different parents. A punnet square would be too big and confusing if we considered all the factors that came from the parents.
Basically you have hydrophilic (water loving) and hydrophobic (water hating) the heads are hydrophilic and the tails are hydrophobic. this allows the cell to be selectively permeable. this means selectively allows or does not allow substances In or out
