[module0000]

This question has an easy answer: exactly what they have been doing for 20-50 years. Inertia is a real thing.

Other relevant answers might include "true fault tolerance". More than half of your mainframe can be burned to a crisp, or being chewed on by a dinosaur - doesn't matter, it will keep working. The closest bad analogy I can think of...is a mainframe is like the NASA space missions of yesteryear. Every component has three or four duplicates, just waiting for a primary component to fail so they can take over. They aren't cost effective, but they are operationally effective.

[smolder]

Other relevent factor: mainframes are probably not riddled with security flaws. If Iran used a mainframe in their nuclear enrichment facility, they might never have gotten hacked by nsa and friends.