I see two likely answers:
Personally I'm going for a bit of both.
Starting over is a lot of work, keeping a category clean is a lot more work than many would think.
Anybody with the skills needed can join the effort, so adding to it isn't that hard.
Those stating it's not used due to low traffic, then why are you trying to get in? Ah, the search engines use it. Those same you claim replaced directories. Taking directories like DMOZ away is also literally taking away part of the basis the search engines are using to determine "crap" from "good". Do it and you'll get even more spam in the search engine results.
The main drawback I see for DMOZ is the utter lack of interest in it from AOL, If I were running it my priority would be to find a new home that cared.