As someone who works for a local refurbisher I have a simple reason for you:
Quite simply, refurbishers are not manufacturers. We fix what we can, and reuse what is mostly good, but we are not in business to hold hands, or to fix every little thing that can go wrong with a piece of equipment. If you want something with a warranty, then buy new, and buy it from someone who has warehouses full of replacement parts, that is all a part of the cost of retail parts. If you want a steal of a deal on something that will get you through until you can afford something better, then buy from a refurbisher. It may last 90 days, it may last 5 years, but that is the simple reality.
The referbisher that I work for in particular is a nonprofit that works with low income families, and we do bend over backwards, and often go well beyond our warranties, but that is not everyone.
That being said, there are things that you should NEVER buy second hand (like HDDs, motherboards, and power supplies), and there are things that are quite safe to buy (like monitors, keys/mice, HSFs, non OC-able CPUs, speakers, etc), but graphics cards are in that grey area. If you do get it, then immediately put it through its paces and make sure that the temps, framerates, and everything is within spec, and if anything seems not right then return it right away.