Because things like lead soldering being banned in the EU which means we have to use cheap tin solder which is crap. RRoD and YLoD are both because of crappy solder.
Tin whiskers seems to have been solved quite some time ago, and they were the only, "oh ****," level lead-free problem.
Most P3, Athlon, Athlon XP, P4, etc., PCs are still going strong, with only minor parts replacements occasionally needed, and with the Athlon XP and P4 came ROHS.
It has been more recent big vendor PCs, with rage-inducing cost cutting, that have had the kind of mid-life failures that current consoles are known for, and for basically the same reasons.
Most
business laptops from years ago are also still kicking along, generally just needing HDDs, batteries, inverters, video cables, etc., replaced, and most of them are Pb-free.
Most
business desktops from several years ago are still going strong, as well, and like the business laptops, tended to go ROHS ASAP, instead of waiting for the final deadlines. I would bet good money that there has a lower failure rate of Thinkcentres and Precisions than of Presarios and Dimensions; likewise the same for Lattitudes and Thinkpads v. Inspirons and Pavillion notebooks; all ROHS compliant.
The thing is, with the above examples, you can just pop them open, and see that the business machines were engineered better: big caps farther from hot components
(one reason I didn't use Optiplexes as an example!), more cooling of minor ICs, more caps and VRMs in general, more metal mass to all coolers, less dead space for cooling, more steel structural support pieces, less reliance on plastic tabs for mechanical integrity, and so on.
Lead-free solder is annoying, but it's not like it can't be worked around. OTOH, trying to cut pennies out of the cost of a several-hundred-dollar box, can and will have dire consequences. Lead-free solder joints may indeed fail, but that is a symptom of poor design and/or not testing in realistic environments (early XB360 and PS3s both faced cooling issues related to where real non-techies put them), more than it is the inherent problems with lead-free solder.
Poor cooling in spite of increasing thermal density will increase failure rates.
Power-saving features that exacerbate stresses from heating and cooling cycles will increase failure rates.
Not being paranoid about heat cycles, with fragile lead-free solder, will increase failure rates.
ROHS adds difficulties, but it's the obsession over price points and power consumption over part longevity, that is the root cause of high failure rates.
Nobody likes lead-free solder, but it is being used primarily as a scapegoat. The real problem is that consumers demand more performance in a small package, and want it cheap. They generally do not understand what that cheapness brings with it, over the long haul. If MS and the like get a big message from consumers that they would rather have part quality, and would actually pay for it, chances are that failure rates would drop like a rock, for the next gen consoles.