Why is non-copyright music not credited by YouTubers?

Since YouTube early age till now, many YouTubers uses non copyright music for their videos. That’s fine but I don’t understand why many of these YouTubers didn’t credit the music for people who want to hear again. Of course there are some YouTubers did credit but many also didn’t credit. So why these people never credit the music? I can guess laziness but isn’t the song good that it deserved to be spread?