The issue is, what is free today, may not be free a year from now. I was using music that I actually bought when I first got serious with YouTube back in 2014. Years later I start getting copyright violations, and emails from copyright tracking services about them. I looked into fighting these claims because I had actually bought the music, but the process was long and tedious and wrought with complex language that made everything as clear as the Mississippi River. It was way easier to just replace the audio track with music from the YouTube library than to fight the false claims. Now, I either create my own music or use music from the YouTube Free Audio Library. Yes, the YT audio library does not have a lot of music I like. 95% of it is midi music that reminds me of playing video games on the NES and very little of it has the correct cinematic sound that fits the flow of my video style, but there are a few real musicians that create music with real instruments for the library.
Also using music in videos is something for me that is not always a crowd-pleaser. People are extremely particular about the kind of music they like. I get a lot of negative comments on videos specifically complaining about the music. Personally, if I start watching a video with loud music that I like, I either mute the video or click off it. I have an extremely narrow taste in music. Pretty much all modern music created in the last 30 years is like listening to cats scratching a chalkboard to me. There are some that I like, but it is not any of the popular stuff. I am an orchestra guy. I like Beethoven, Mozart, Chopin, Chikauski, that kind of music. But I understand that I am me and that not everyone is me, and that is okay. I have never left a negative comment because I did not like the music on someone's YouTube video. I just do not watch it or I mute it.
My point here is that, using music in a video is a dividing factor. I have never received a negative comment because a video did not have music, but I have received countless negative, sometimes vitriolic comments, because it did. I still use music on occasion because the video just calls for it, but it is not a standard of practice for me. If I use music, I either create it myself (I have two sons who are skilled guitarists) or I use music from the YT audio library.