The problem is that the legacy applications running on that mainframe are tightly-coupled and thus a bitch to test. They're also hard to automate and hard to scale, because design choices made in the 80s and 90s based on people using them at the speed of manually typing on keyboards fall down when you hook up APIs to them for automated testing and for connections to modern applications.

And most of the people who know how to develop for them are about to retire. When was the last time you heard of a modern CS or IT degree program offering a COBOL elective?

And they're not easy to migrate, because you have 40+ years of business logic locked up in that legacy COBOL application, many of the developers of which are retired or flat-out dead. Did they document it perfectly? LOL! And oh, we're dealing with customs brokerage around the world and also potentially accounting info for a publicly-traded company, which means if you fuck up badly enough, people potentially go to jail.