As I understand it, Google (and others?) rank pages higher if the time spent on them is high. Long text takes longer to read, so it's a way to increase that metric.

Another explanation I've seen is for ranking longer texts higher is that longer text is assumed to be of higher quality because it's more content/took more work to produce/whatever.

Whatever the reason, this leads to the rambling articles I mentioned, which seems to be a trend even for cases when the actual useful content is short.