These days something like MusicBrainz is effectively a legacy system. So few people buy CDs anymore that there's not a lot of interest in maintaining it. It's fairly hard to even find a computer with an optical disk reader these days, especially if you are looking at laptops.

Note that the scope of the project goes beyond CDs, it's a catalogue for pretty much any format where you can play music.

It's used as the basis in a _lot_ of places. So fixing errors fixes them in a lot of other websites (and infoboxes).

Never worked fine for me, at least not fine enough to trust it.