I never worked in those ERP companies, but a few times I've been on the receiving end, working at companies undergoing a large migration.

It is very often stuff that doesn't really matter, is highly inefficient, and requires small changes everywhere in the system. Death by thousand papercuts.

There is no incentive from both sides to change: the company wants to keep modifying to get $$$, employees don't want to change how they work (because change is often stressful), and the person paying the bills is not getting the full picture.

If there's anything that is actually really "unique" (in a good way), then you spend money. Often this means not customizing the ERP, but actually writing new software that integrates with it.