They did. Chrystler and alco locomotive works made them.
something about the metalurgy of the cast armor being stronger than the sheet welded armor.
yes lots of work to build. But, maybe it was faster to cast the front and weld it to a body than to have to weld and fabricate the front. Especially on the early small hatch shermans where the front slab could be as many as 5 or 6 pieces of slab and cast hoods.