Why are 5 Volt 7-8 Amps chargers not available for charging phones quickly?
Lithium-ion cells can be made to handle charging rates of up to 5 times their capacity, so theoretically you could charge a 1600mAh battery at 8A. However its lifespan may be reduced, and the chances of something going wrong and causing a fire are greater. All the associated circuitry in the phone (including charge controller, battery protection circuit, wiring, connectors) would have to be beefed up to handle the higher current safely. That is bound to make the phone (and its power supply) bulkier, heavier, and more expensive. 

The other problem is that it wouldn't charge that much faster anyway. The battery's internal resistance limits how long it can be charged for at maximum rate. Topping off then takes almost as long as it would at a lower rate. So while you might think that at 8A your 1600mAh battery would reach full charge in just 12 minutes, in practice it might take 30 minutes or more. 

Also, as a Lithium-ion battery ages its internal resistance increases, causing it to reach peak voltage sooner and take longer to charge. So to keep getting the faster charging time the battery would have to be replaced more often. That is a recipe for customer dissatisfaction.