[xvilka]

Gigantic part of products like SAP etc (maybe the main part) is their integration with local standards and legislation in every jurisdiction the company operates. Yes, writing code that can substitute their products is probably easy. Knowing what exactly to write is hard, especially for multinationals.

[gmueckl]

Another big part is the under the hood flexibility, at least in SAP. What you usually see as a user is a specific configuration of the package that has been adapted to your company's workflows. There is a lot of domain knowledge and implementation effort hidden within that configurability.