Many times, doing an excellent job means putting all the infrastructure in place and make enough progress to make your job obsolete. Current employers often ask you to show your value first, but by the time you've actually made your own work obsolete, their short-term memory never see you for what you have done and will try to evaluate you as you are now and what can you still do for them. Essentially they never wanted to pay you for the value you’ve provided, they would rather find someone new, whom they potentially know the least about.