However, it is impossible for ER P software's original programmers to design software that covers the needs of every single large business. Every business is different. They manufacture different products. Their employees have different titles. Their programmers have different computer jobs.
Inventory control is quite different for cars than gasoline, for bushels of corn than software.
Therefore, every package must be extensively adapted to each client. And since businesses are not static, they change, the software will not stay the same for long. It must be tweaked, revised, cut and expanded to meet the ongoing needs of the business. Actually working with the software as it stands, and developing applications for your particular business. The bad news is that all this training is expensive.