Why most appliances in US don't have third prong
Many appliances in the USA and elsewhere have two-prong plugs because they are "double insulated." The third prong is for ground fault protection except where outlets have been designed with protective shutters on the current-carrying slots that are opened by the ground prong. Double insulation provides very effective ground fault protection that is less expensive for most products.

The fact that the US voltage for domestic wall-plug use is 120 volts offers a small amount of additional safety vs. 220 volts, but that is generally not taken into consideration in US safety standards.

Double insulated or class 2 electrical appliances are products that have been designed in a way so as not to require a safety connection to electrical earth (These products must NOT have a safety connection to Earth).

These products are required to prevent any failure from resulting in dangerous voltage levels becoming exposed causing a shock etc. This must be done without the aid of an earthed metal casing. Ways of achieving this include double layers of insulating material or reinforced insulation protecting any live parts of the fitting.

There are also strict requirements relating to the maximum insulation resistance and leakage to any functional earth or signal connections of such appliances. Products of this type are required to be labelled "Class 2," "Class II,", "double insulated" or bear the double insulation (square in a square) symbol.

The safety of electrical product designs are certified by an independent testing agency such as Underwriters Laboratories (UL) or the Intertek ETL laboratory (originally Edison Testing Laboratory). In order to apply the UL or ETL label, the manufacturer must have product design evaluated and samples tested in the independent laboratory or have their own testing and review results evaluated by the independent laboratory. In either case, they must maintain a contract with the independent testing laboratory that includes periodic, un-announced inspection of their manufacturing facility and its quality assurance procedures.

See the product marking illustration below.