Because after spending years building it out, the company will be loathe to redo all the work, especially since it is so tightly integrated with the third party

Because after spending years building it out, the company will be loathe to redo all the work, especially since it is so tightly integrated with the third party system. They won’t replace it until they replace the third party system someday, but because they are just now starting to use the third party system, it will be many years before they abandon it to move on. I’ve worked at large companies for decades. Switching major systems like these takes many many years. If you can get involved with setting it up and maintaining it, you are almost guaranteed job security for 10-15 years.