Yeah, unfortunately belaying is one of those things where it's incredibly easy to lose focus, and when you do the consequences can be catastrophic.
Unfortunately, climbing in general can be like this. Maybe you get reckless and space out your pro too far. Or you forget to tie a knot in the end of your belay line. The more competent you get, the easier it is to get too comfortable and to make a truly catastrophic mistake.
But I think most of the "dropped by belayer" incidents are caused by belayers who were taught improper technique, or never taught at all. I've seen lots of youtube "how to belay" videos that involve switching hands and other dangerous practices which involve taking the brake hand off. Just slip slap slide, people.
Unfortunately, climbing in general can be like this. Maybe you get reckless and space out your pro too far. Or you forget to tie a knot in the end of your belay line. The more competent you get, the easier it is to get too comfortable and to make a truly catastrophic mistake.