It's just IBM marketing :-) To the extent it's true, it's because the people that have built these systems are experienced and have a lot of business knowledge

It's just IBM marketing :-) To the extent it's true, it's because the people that have built these systems are experienced and have a lot of business knowledge, and know how to reuse and tweak their systems while not trying to build each new requirement with the latest interesting framework they downloaded 5 minutes ago. But it's slow to code in, especially if you go outside COBOLs comfort zone.