Noticed the same thing, must be to do with licensing.
I've seen these songs missing on other streaming services. So it's definitely a rights issue with people refusing to license those particular tracks from albums for streaming. It is annoying. It's more annoying later though when tracks that used to be on Apple Music get removed, and you can no longer play them. And it will happen, I've been paying for streaming for years and the problem has existed on every single service. My Spotify playlists all have more than dozens of tracks that are no longer playable due to licensing issues.