I think just the size of the projects. My employer has had a team of ten-ish people working full time on porting a piece of our COBOL program (the full thing was deemed not possible to port to a single modern application, so it's getting split up) for about a year now. I was peripherally involved in four rounds of increasingly integrated testing environments. Go live was yesterday. They rolled it back three hours in.

It's hard for a company to invest multiple people's salaries for many years on reworking software that currently mostly works.