When the first alien visits the Earth, it lands in a cow pasture, not Washington DC. It doesn’t ask, “Take me to your leader,” but rather, “Would you accept this pound of gold (whatever!) for that cowpat over there?”
Other members of the alien race visit the Earth, looking to acquire cowpats. Not just any cowpats, only the most artistic cowpats. Any and all attempts by government dignitaries or scientific specialists are rebuffed by the aliens.
An industry arises, the sole purpose of which is to determine which cowpats will bring in the most money. Just as humans are starting to understand the artistic proclivities of the alien species, they stop visiting the Earth, and the Earth’s economy, strained at first by switching to a new standard, is now crippled when the bottom falls out of the market.
Until a new alien race visits Earth to find bargains on (IIRC) used chewing gum.