Because curation to keep spam out would be work. And how do you prevent trojan horses, where an initially promising blog is turned over to SEO once included in the official dmoz?

Could be done, but it'd take time and effort.

There’s also the issue of quality. Do you just include everything that is a blog, no matter how low quality it is?

And if the answer to that is no, then where do you draw the line when it comes to quality?

Curating anything is complicated and different people will have different opinions which is why you end up with different lists curated by different groups of people with different ideas about what should and shouldn’t be included.