It is a very confusing subject. Part of it comes down to, "Why would unsold tea be kept?" If you take the internet at face value, Taiwan is home to warehouse after warehouse stuffed full of decades-old oolong tea that was never sold or thrown away, despite being a populous island with a fixed amount of space. There are also the typical costs involved with warehousing any product that would surely add up over time, protecting the tea from pests and elements. I am unfamiliar with government policy in regards to tea in Taiwan, but unless they were being paid to keep it, it is hard for me to understand why they would have if there was no expectation of it being sold at the time.
Another part is, "How would someone find this unsold tea?" To me, this must mean that some of this tea was not in warehouses but in teashops, where space is at even more of a premium. Of course, not all teashops are run in a rational way (MarshalN's series of blog posts on his 2007 vacation to Taiwan which he seemed to spend almost entirely hunting for old oolong was helpful to me for providing context), but normally businesses discard what they can't sell unless it serves a purpose, such as for advertising.
A third part is, "Why would anyone even want this? Why would they look for this when there is fresh tea available every season, as there is in Taiwan?" A very helpful bit here was a post from Stephane in 2005, when drinking this type of old oolong was still new enough that it needed to be explained:
http://teamasters.blogspot.com/2005/09/ ... young.html The speaker sees the desire for old oolongs as a nationalist counter against pu'er tea.
Then the final part, "Some of this tea is not being reroasted, but is still being bought; are people just drinking stale tea, or is something else going on?" which goes back to what is happening when tea goes stale, and why something like this could even be possible, which goes back to the enzymatic reactions in the tea, since these old unroasted oolongs are apparently not post-fermented teas, have nothing preventing them from going flat such as re-firing, and yet are still apparently in OK shape.