The Airbus A380 seats around 850 passengers 4. American Airlines has a fleet of about 950 aircraft of various models 2. The Cadillac of Wal-mart life-vests, purchased individually, costs \$30 1. Outfitting a fleet of 950 A380s with life-vests purchased individually at Wal-mart comes in a $24,225,000 USD. Even though I outfitted one of the largest fleets consisting of the largest aircraft with the life vests purchased using idiotically low business-sense, if I managed the rest of the business with some sense then the cost is (optimistically) about a day's worth of profits (American Airlines had a net profit of \$7.6B in 2015, though 2016 was a little slower at \$2.7B 3) for a once-in-an-aircraft's-liftime purchase (though there's no reason to retire them with the aircraft).
Regardless, your question was "why", not "why not"; what incentives are there for doing having life vests? One possible incentive for airliners to go ahead and equip all aircraft with them is to take out any restrictions on the aircraft's serviceable zone that would be in-place if not equipped with life vests. Maybe airports such as New York require life vests because it's so close to a large body of water that traveling over it is practically unavoidable? Maybe it's economically advantageous to be able to re-route aircraft without the luxury of predictability.
Still I apologize I still have not fed you with any true answers, only supposition. Maybe there is a regulation...Otherwise, maybe the CEO of American Airlines will respond and tell us why the hell he blankets the life vests.
(Edited for coherence).
Mins raised a point about cost of maintenance being a significant cost (possibly larger than the cost to supply). So maybe the there isn't much strength in the "why not" court.