The answer is that weight considerations are of minor concern to a stationary plant. There is no advantage in a lightweight fuel to an organism that is not going anywhere. The chemistry of assembling starch chains from glucose and recovering glucose again is particularly simple, whereas the metabolism of fats and fatty acids is more complicated.

Plants store energy in starch because it is especially easy to get the energy in and out. Animals, which must move about and carry their fuel supplies with them, find the low energy-per-gram feature of starch to be a disadvantage. They put up with the more complicated chemistry of fats to achieve an energy-storage efficiency only slightly less than that of gasoline.