If you're building a space ship you have very tight tolerances and specifics needed for the alloying of the metals you use. The aluminum in aircraft is full of strange things that need to be removed for general use aluminum and definitely need to be removed for building new aircraft.
You're right that melting and re-casting aluminum is really cheap and efficient for beer cans, it's not so much the same for aerospace aluminum.
When it comes down to spaceships, the energy used to produce them is usually a very small portion of the total cost of the mission and we don't make very many of them – recycling isn't and shouldn't be a priority.
The Falcon 9 burns 110,000 liters of kerosene every launch, if it were made of carbon fiber, maybe it wouldn't matter how the structure was disposed.