What is cause of frequent starter failures in my vehicle?

What is frequent,how many starters have been installed? If more than one,time to opt for a better part.

I would get a new mechanic because brushes don't "stick"! They can wear out and no longer make contact. The bendix may stick. As wetry said it's best to pay for a better quality part. Most of the time you can get a new starter for @ $20 more than a re-manufactured.

sounds like poss cheap starter and a line of bull

Also the Oil Filter is mounted directly over-top of the starter,which is a very bad design,and when you change the filter it leaks oil all down in the starter if you don't put something over the starter to defer the oil from going on it,like place a piece of card-board over it.it will continue to go into the starter every time you change it and that will lead to starter problems as well...I'd check to see if maybe you have a oil leak in that particular area allowing oil to run on/in the starter...Hope this helps.

If having repeated unsolved starting problems try replacing the starter motor relay. In the 03 Sable it's #23 in Power Dist box under hood. About $15.