There were multiple reasons for this, most importantly the SSMEs were routinely used well past their rated thrust efficiency (you hear shuttle pilots being ordered to go to "104%" or higher on most launches) which essentially requires the engines to be completely overhauled and rebuilt after every flight.
Would you have any source for that? Shuttle main engines at 104% were not running beyond their spec. The final design of the SSME was capable of somewhat more thrust than originally specced, but for the sake of convenience "100%" throttle was calibrated to be the same amount of thrust as 100% of the original intended spec. IIRC the true maximum was 109%, which was indeed beyond normal safe limits and only used for emergencies.
Much of the excitement we get out of our work is that we don't really know what we are doing. -- E. Dijkstra