The "insert" format's portable so we don't need to massage the data twice when we update it. For instance during the so-called "energy crisis" last summer there was talk in California of going to double-daylight savings time. That would've required new files to be generated (and screwed up all timezone data in the world which uses Los Angeles as a surrogate for the Pacific Timezone!)
So we're trading off speed of loading for easier maintainability and of course an easier porting task if we adopt a third RDBMS in the future.
