The starter overrunning clutch on most starters is fairly simple to replace, but why is it seldom replaced?

more often than not the part by itself cant be bought, they do sell starter rebuilding kits aswell as alternator, but the cost being around the starter or alternator itself makes it more appealing to buy a whole new one. also its cheap insurance to that the starter wont break down because of something else again. i do remember my dad telling me that when he used to work on cars he would rebuild starters and alternators and reuse the working components and fix the ones that are broken because of the fact that a brand new starter was so expensive back in the day.