Magnesium in theory is great, and probably great for single season use for a pro, but the long term problems are tricky. It corrodes, it also undergoes electrolysis if another metal touches it. It's difficult to machine or cut or weld. I don't care what they plate it with, if it chips the finish, it will corrode fast, especially in coastal air. New alloys are not flammable at least.
My guess is for the minor increment in performance, it's not worth it.

Here's a bike with bamboo tubing. whatever...