A thousand times this. If you wonder why beige Pentium II 233MHz systems are going for $200+ on ebay, it's because people who are 35 now and having nostalgia for their youth were 15 back then and playing games on them.
It's got to be more than that for them to command such high prices though.
Many people grew up playing games on a NES, but they don't go for that much money. I picked one up last year for $30 including shipping.
I think the other factor is availability. PC's were pretty much a commodity by that time, and could also be upgraded.
The older machines were superseded by newer hardware that could still play all the games you played on your old hardware - essentially the same machine but faster and better.
Why would you keep your old one?
Eventually those machines would be handed down to friends and family until a new version of Windows came out that wouldn't run well enough on the old machine, and then it would end up in landfill.
An NES will still play all the games released for it, but PC's underwent forced obsolescence.
I think they go for high prices because of two factors; nostalgia and rarity. They were extremely common when new, but not many have survived.
Looking at the PC's available on ebay, it looks like a lot of them were office machines. That makes sense, because businesses will often upgrade only when they absolutely have to, and it's also difficult and expensive to dispose of a large amount of computer equipment.
My small collection of old PC's came from a skid in the warehouse at my job, where they had been stored for a couple of decades.
I was given the task of arranging for it to be recycled, which I will have to do because I've taken as many home as I have room for (or that my wife has decided we have room for :D ).
I bet a lot of those ebay machines either come from people like me who's boss lets them take them home, or from recycling centers that get lots of machines from businesses.
I bet hardly any of them have actually been stored in peoples homes this whole time - those probably got discarded decades ago, just like my first Pentium 1 machine