Because it's buggy? Seriously, I've been using Nero for years, and 6.6.x.x has given me more problems doing basic burns than all other versions put together, let alone the other bugs Nero itself has admitted to over the last few versions.
I know I was using 80 minute discs and the data wasn't too large, but I've still seen this error on different discs, different drives - and I'm not the only one reporting this kind of problem. As you said, many people have issues with Nero.
Maybe you've just been lucky and your hardware matches what Nero use themselves or you don't use the program as extensively on DAO burns, but that doesn't mean a lot of us arn't seeing basic problems with the current Nero. Problems that I didn't see with the 5.x version or the early 6.x releases.