The other factor is sometimes, people ask for silly money because they don’t want to do the gig. Rather than say no, just ask for insane amounts. I suspect DW along with JKR could fill their entire year with school visits. He probably allocates x days for publicity including schools - after those are filled, its his standard corporate rate. And if people are stupid enough to pay it.
At the other end, I also have an issue with expecting authors to work for free - school events take time. Done properly, an author will prepare the event and use at least a half day including travel. Often, the event won’t instantly result in sales. Most authors do not earn enough to live off including most children's authors. It simply means that more privileged people can afford to do the working for free bit and exclude authors who have to work to pay the bills.