Usually the word "custom" would be used in conjunction with the idea of something special. For instance, the concept of having suits customized and tailored specifically for you. Customizing a home to your tastes would be considered special and unique. The point behind customizing is having something that is specific and specialized for your taste.
The other side of customization is that it is typically expensive. That tailored suit or customized home is incredible, but it cost more than a suit off the rack or the home that was chosen from the builder's stock plans.
It used to be that in order to get an ERP system that would work for your company that it had to be highly customized, and that usually meant really expensive. Not only was it expensive to implement, but it has been expensive to maintain. With every product change, upgrade, or software add on, more money has to be allocated to update the customized ERP system.
With a lot of these customized systems aging, and becoming less effective, it is hard to imagine letting it go, even though the time has come. This is a resource that a lot of money has been dedicated to, and the thought of implementing and maintaining a whole new system isn't all that appealing for some.
Consider this though: Some ERP systems have become much more flexible and require little to no customizing. Functionality out of the box. It is a concept that is difficult for many to fathom, but ERP technology has come that far.