Because building AP may help your shield and your heals more powerful, but unless you have a large amount of income coming in (getting kills or lots of minion kills, which don't happen for supports), it will scale very slowly. The damage you will put out will be very minimal and the shields and the heals will not be much stronger than the base. Even when you do build AP, the items you build are very weak and do not help your abilities as much as they could. If you go AP, go all out. Get a deathcap as your second or third item. Otherwise it's not worth it. You either go full support or full ap. Never in between. The benefits of going full support is giving the team extra auras and utility. Support masteries give you shorter cool downs on summoner spells and abilities. Getting items like sherylias reverie and randuins omen give you activation spells that benefit the whole team. Aura items like frozen heart, aegis of the legion, and will of the ancients give the whole team useful bonuses. All of these items are cheaper and easier to obtain when using gold per ten items and not farming. It makes getting wards a lot More reasonable too. Going full support is definitely more reasonable and is overall more useful to the team.