The simple reality is that clients who are given source files as a “bonus” can and, often times, will take your source files to another less expensive vendor to complete the work.  It isn’t as though clients have limited options.  You have done all the groundwork and “created” the process for someone else to finish more cheaply.

The result of your generosity is that it allows a less capable designer to take on a new project without having to do ANY of the conceptual work or product development.  Then, in addition to getting to bill the client for work they didn’t create, they also get to learn how to do the work from your source files.