Main reason is, most gas is used up within a relatively short amount of time. For example, the gas stations might get a load of new fuel every month, people normally use the fuel in their cars up within two weeks or less, ect. Really no reason to add stabilizer to all gas if very little of it is kept for long term storage, all that would do is jack up the price. Diesel/Fuel-oil on the other hand, does get more additives in the winter to help it last longer and to prevent thickening when cold, as well as growing microbes.