I've also noticed that they are generally poorly made. It's a shame because it reflects poorly on the technology when it's clearly not a problem with the diodes themselves. If someone would make a set with heavier-gauge wire and better, more weather tight connections, I see no reason they couldn't last more than 10 years without issue. Of course, that may be why we'll never see a set like that. I've actually contemplated making a set like that for myself, but I don't know what I'd use for covers.
I have an indoor chaser set that has had two breaks in three years. One at the controller that I was able to re-solder, and another somewhere in the wire near the transformer that I'm unsure if I'll be able to fix without putting a new connector on it.