No question that word counts re genre are guides, and that there are exceptions. I would say more so for adult books than for PB, MG, and YA, but there are exceptions for the latter group as well.
Although it's fairly obvious, I don't think anyone has pointed out why. The more words, the more pages in a book. The more pages the higher the production cost. The higher the production cost the higher the retail price. The suggested word count for children's books allows publishers to bring in a book at a cost that enables them to retail a book at a price point they think is most desirable, and one that they believe will produce the most sales. Of course this is a generalization, and as said, there will always be exceptions. But I think that to be an exception a book has to be considered a "can't miss."
There's another reason as well, and that's from the kid perspective. A fluent adult reader, college-level, reads upwards of 300 words a minute when reading silently. A third-grader is considered on track if he/she can read above 100 words a minute. So, do the math. 30,000 words divided by 100 words per minute equals 300 minutes of reading time. 300 minutes equals 5 hours of solid reading to finish that book. When you consider that the average modern child reads about 20 minutes a day, if we're lucky... That's at least 15 days of reading to finish a single book. Every 1000 words you add to the length equals another 3 days of reading.
Of course there are love-to-read kids who read more than 20 minutes a day. And fluent kids who read 200 words in a minute. But...are you willing to stake your career on building them into your audience? Or are you aiming for something a little broader?
It isn't anything negative about the modern world. Kids actually read for fun far more now than they did 20 or 30 years ago. But they also have homework and soccer practice and chores and stuff. For a while, my younger son was trying to keep one book at his dad's house, one book at my house and one book at school so he wouldn't lose anything. So take that 15 days of reading, split it between three different books, and suddenly you have a kid who takes 2 months to finish each book, even though he loves to read and reads every day.
I know so many struggling and reluctant kids who will take one look at a book with a two-inch spine and never even read the back cover. Just the size is enough to scare them.
And many schools reward kids for the number of books they've read, not the number of pages....
Plus teachers who purchase a class set of books aren't going to take on a monster tome that half the class won't be able to finish. They will assign a chapter a day...so keep those under 20!
It all adds up to: If your story demands a higher wordcount, then so be it. But there are definite advantages to keeping it small.