Because they decided to name things -ium to sound alike later down the road, even if it’s etymologically incorrect.

If we used the original etymologically correct names for elements we would also say plumbum, ferrum, argentum, aurum, cuprum, stannum, and hydrargyrum for lead, iron, silver, gold, copper, tin, and mercury respectively. Which is why their element symbols are Pb, Fe, Ag, Au, Cu, Sn, and Hg.