This is not unique to HDDs, or to any backup media. Quite the contrary, actually.
However, when people use HDD "backups", they seem to stick to the second half of their mirror as the backup. So, essentially, you have one backup medium for your data, not two. That is, original HDD and mirror HDD. Dual DVDs means that you have the original HDD plus two DVD copies. I admit that this wasn't clear from my post.
But there's another important difference between HDDs and optical media, is that the HDDs are more exposed to OS write errors, or problems from a faulty power supply, motherboard, controller, electrical black- or brown-outs, etc., since they're not WORM (write once, read many) media.
When I wrote that HDDs have a high probability of failing within a short time of eachother, that was not a joke. This has to do with the drives having very similar runtimes and working conditions. It is also likely that the drives were purchased at the same time, and therefore from the same production lot, which increases the risk of similar production anomalies.
With WORM media, it is common for you to, well, write once, verify the integrity, and then "forget it". This increases the likelyhood of detecting similar production anomalies in the media, compared to HDDs. What you cannot detect, of course, is how the media deteriorates with time; is the plastic acid free and does it seal well enough, is the dye pure and of a time-resistant type (e.g. advanced metal AZO) or not (cyanine or phthalocyanine), and so on.
I'd argue this issue is even more prevalent for DVDs/CD-ROMs, as people tend to burn a DVD, tug it in a pouch and forget it. If they need that backup years later they might see bitrot has taken its toll - this might be the case even with redundant DVD backups.
With HDDs you are generally copying data to the same HDD, thus ensuring its integrity every time you do so. How many people backing up on DVDs check their integrity periodically?
How many people backing up to HDD check their integrity periodically? I think the proportion of people verifying their backups are similar, regardless of media.