Sometimes they're in a playlist, but sometimes I'm not watching via the playlist. Like if I know that episode 19 is the next one I need to watch, I go in search and find "Super cool series episode 19", and then I watch it.

I don't search for the playlist, then go to the playlist and scroll through to the episode I want, then pick it.

But then since I didn't go to the playlist, now YouTube can't recommend the incredibly obvious next video in the series.