Why are light fixture wires allowed to be lighter gauge than house wiring on the same circuit?
The wires to the light fixture will only carry the current drawn by that particular light fixture (on the order of 1A or less, for simple fixtures). The wires in the wall between fixtures carry current for all fixtures on the circuit, and are protected by a fuse or circuit breaker in the main panel. The wires in the wall must thus be sized to carry the maximum current allowed by that circuit breaker. For 15A circuits, this is #14 wire; for 20A circuits, it is #12 wire.

Note that the wires to the fixture must also be sized to carry the maximum expected current, but that is (largely) determined by the manufacturer of the fixture. Light fixtures have a sticker somewhere stating the maximum wattage of bulb that can be installed. This is partially due to heat (not really a concern anymore with CFL and LED bulbs) and partially due to the size of the wiring.

Some (better quality) fixtures / equipment will also have internal fuses or breakers that are sized to protect the wiring in the cord.