Except that gold tends to concentrate when it washed around with water. Thus gold in asteroids may be too diffuse for easy access- not to mention mining on a asteroid could be very difficult and expensive.
There’s a line in the movie that says the aliens specifically think of humans as insects. If you saw a diamond sticking out of a termite mound, would you bother giving them something in return before taking it?
I suppose it’s possible, but when you factor in the concept that a bunch of undeveloped yokels with primitive weapons pose a significant enough threat to your production (not to mention your lives), take the easier route and just mine the damned asteroids. Or come in sufficient force to quash the natives. That’s pretty much what the Spanish did in the New World.
I guess ET is smart enough to cross the stars but not as smart as a bunch of 16th century Europeans.
Maybe I’m wrong and this movie makes absolute sense (upside down steamboats and all), but I’m guessing we’re not supposed to think too hard about it and enjoy the special effects.