replaced my Alternator four times in six years

it seems in checking my receipts, I have put a new alternator in my truck every eighteen months or so. this is the fourth time in six years. I drive less than ten thousand miles a year.
it is an all stock original unaltered truck. are the replacements that bad? is the grounding poor? if there is a problem with the wiring or? why does it not fail immediately ? instead of taking so long? this is getting old.