Replacing a home brew ORM in an application of any scale is a massive time and bug investment. It will be tied into every single aspect of everything, and even into the build and database deploy processes. It often has its own classes with their own internal behaviour which get passed around raw - so you can't just upgrade one price of code and test and move on, you'll have to rewrite the entire product at once.
Having seen it done I can understand very easily how someone could not just remove it.