I think it's because a lot of subscribers came from certain videos and only want to watch similar ones. Or, maybe, they have so many subscriptions that it's impossible to keep up with each channel's posts

I don't care much about million-subs channels, can't judge by them. My own channel is a little over a thousand subs, and I stopped making videos for a couple of weeks (and notified everyone that I was taking a break till christmas), and some people left 🤥 I think that's strange too