Sure, secondary containment might fail. But it buys a second chance and requires penetration of meters thick reinforced concrete on top of a failure of the pressure vessel. With such secondary containment it's likely that Chernobyl would have only resulted in the deaths of operators working near the reactor and water supply, and drastically less contamination of the surrounding areas.

And research papers dispute the risk posed by said bubble: https://www.sciencedirect.com/science/article/pii/0146572483...