Their thinking is that their best shot for readership is to stay around 1.5K words, for every chapter and if you go longer they'll get bored and too short, the readers will feel cheated.
While there is a grain of salt to this idea, I love variety in my chapter lengths as a reader.
My favorite bits of "logic" circling these groups is to "load up" your first free chapters (which are free) to draw readers in and cut the word count in half for the rest of the series. (Think 3.5K to 1.5K words)
The other bit is that it's better to widdle down your word count to fit their guidelines and loose money instead of dividing the section into smaller chapters. These "edits" come after your regular editing. One newer writer had a 6k word chapter (yikes) and instead of just splitting it into smaller increments like a lot of writers here do, one lady (same lady who blocked @therosesword and I) encouraged the writer to scrap a good 75% of it to get it down to the "sweet spot".