Let's refine your calculations a bit. Worldwide, less than 200 rockets are launched each year. They have an average empty weight of 30 tons (rounding up a bit), which is mostly aluminium. That adds up to 6000 tons.
This is a tiny fraction (0.001%) of total aluminium production (60 million tons).
if it were economically feasible to recover rocket stages for their scrap value, it would be done already. In fact, when a rocket is launched over land (all Russian and most Chinese launches), the spent stages are routinely recovered and scrapped. Recovery from the ocean is far more expensive than the value of the metals, so this is not done.
the fuel is more valuable than the metals. A rocket with an empty weight of 30 tons contains ~500-1000 tons of fuel and oxidizer. There's no way to recover this.
A single Ariane 5 contains ~ 500 tons of solid fuel, which contains 20% aluminium. I suspect more aluminium is used in solid fuel than in rocket structures.
we routinely throw away far more than 6000 tons/year. Ships sink and are left to rust away. Containers wash overboard. Metals including aluminium foil go into landfills, etc. etc.
And about the cost being misleading: Aluminium costs € 1500/tonne, so there's € 45000 worth of aluminium in a launcher. The launcher costs € 60M, so the materials costs is dwarfed by the cost of R&D and labour.