But what this hurts is the small to mid-size shops that cannot afford in-house employees for their various IT services.
Part of contracting is handing over the completed work and being 'done.'
Be professional and work with the new person to answer questions they may have.
Maintenance is easier than development (or at least it should be) so using less-experienced people in that position can make sense.
Handing over the project leaves you free to move forward to ever newer and more interesting projects.
Part of the joy of contract work is learning and using new tools.