And they did make magnesium decks. They are even lighter than the aluminum decks and just as durable. I don't know why they stopped making them !
Magnesium is 3 times the price of aluminium, good enough reason ?
It also costs 3 times as much to machine as it has a nasty habit of catching fire which can only be put out by smothering with sand or lead oxide.
And yes it burns under water, Those underwater flares you used to see being used in every second programme of SEA HUNT ( shows my age ) were magnesium.
White distress flares used to be magnesium as well.