Cost? Don't buy it (no pun intended).
After thinking about it for a bit I'd assume Apple doesn't want to include it because of how big the adapter is.
Also, I just got off the phone with Apple and the rep advised me not to use the IPad charger over a long period of time as it is not good for the battery. I feel like we've seen Apple say the opposite in the past and their support site even lists the iPad charger as being compatible with iPhones.
Not sure who to believe.
Let's examine the situation. Go with a published statement by Apple. Vetted by engineering and legal. Something you can go back to as proof. Or the verbal statement by one rep over the phone. That may have been hired last week and gave you their opinion on something they have no training or technical knowledge of and contradicts apple's own corporate statement. Mmmmmm let me think.......
Does that clear it up for you?
As a side note, the charger is built into the phone. The power adapter cube is a transformer and rectifier that turns 120V 60Hz and 220V 50Hz into 5V DC. The charging circuit, in the phone, controls the amount of current drawn from the "power adapter" small cube, or iPad, or the even larger 28 watt power adapter used with Mac which needs a different cable to use for the phone. I use all three.
The charger inside the phone regulates the amount of draw from the power adapter regardless of how big it is. From 0 to 70% it will draw maximum the power adapter can put out. Then progressively steps down until at 100% it is trickle charging intermittently. At no time does the charger allow the Li Ion battery to reach high damaging voltage. According to some sources, Battery University, and other articles, it may even be better for Li Ion battery's longevity to charge at faster rate.
It is yet to be revealed what the cause of Samsung's exploding battery is. The general statements keeps saying battery, which it may be. But it could alternately be a faulty charging circuit in the phone that allows too much current and goes over voltage. ALL Li Ion batteries will exhibit the runaway explosive reaction if taken to an over voltage state.
As to why Apple has not included the larger capacity adapter, do the numbers. Apple has sold one billion iPhones, if the larger adapter costs $3 more that would mean $3,000,000,000 less profit. Every cent counts when you talk a billion units. What could you do with an extra 3 billion dollars ?