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.