Your focus on containment is completely misplaced. Nuclear industry as well as the wider process industry relies on defence in depth. Blaming cheaping out on containment is like blaming your bandaid instead of using a full bandage when your body has broken out with sores due to herpes.
The focus is on preventing incidents, not mitigating them. That's not to say that both didn't fuck up there too, Fukushima's emergency cooling design was just stupid, and Chernobyl actually allowed operators to do something truly fucking dumb at the direction of someone dumb asking them to do the dumb thing (there's a reason safety systems take control away from people, people are fucking dumb).
Containment is a mitigation. If you are relying on a mitigation measure you've already beyond fucked up.