Catalytic Convertor problems of the 2002 Lexus ES

This is the 2nd 2002 Lexus Es300 ive owned with half the miles of the 1st one. . . Yet both cars catalytic convertors failed!! how is that possible at only 136k miles the catalytic convertor needs to be replaced!! the 1st car had almost 268k miles which I felt the catalytic convertor failure wasnt as surprising. . . But between my boyfriend and I we have owned 4 Lexus vehicles: 1997 Es300 1999 rx300 and 2 Es300 and all of them had catalytic convertor failures!!! I was told by a mechanic as long as you put in the correct fuel and correct oil the convertors should last forever!! so why is it that Lexus cars convertors fail as low as 136k miles!! thats rediculous!!.