Well, partially because in some cases it isn't the same hardware! There are mainframe machines built to run COBOL programs efficiently, like IBMs Z Machines. In those scenarios, you'd likely have something like a standard Linux server as your API front-end forwarding requests to the COBOL machine.

And what makes them differ? Well, the CPU has dedicated instructions for certain fixed point operations. For a given request it's only going to be a few ns faster, but when the vast majority of your workload is performing those actions, it adds up quick.

Another issue is rounding error. With Fixed Point numbers, you still have to round off partial results and the rules for rounding are surprisingly complex. So when you port from COBOL to Java, you need to ensure you port the rounding rules too, or you'll get different results when you rerun last month's reports. No bueno.

Anyway, all this is not to say that COBOL is better or worse than any other language, just that its primitives differ in behavior from other languages in important ways that can make it hard to migrate.