Fan and turbine failures are supposed to be contained by the engine casing - it's part of certification. But as we see here, uncontained failures do occur. In general "airplane builders" do the absolute minimum to meet certification so they're not going to add reinforcement to protect against such an event until they're forced Into doing so by the authorities.

Turboprops can't, of course, contain a propellor failure which is why they have a big slab of armour in line with the prop disk. So in that case, yes, safety wins over cost and weight.