I recently did a livestream showing why 32 pages is the standard for picture books, and what this means for picture book creators. In case it helps anyone, the archived version is above.
Basic reason: 32 pages can all be printed on a single sheet of paper, making it cost-effective. It’s possible to make a book any number of pages, but anything non-standard tends to cost the publisher more money to print.
Sometimes the publisher will opt to do a longer book (my books with Simon & Schuster are mostly self-ended 40-page picture books), but I leave that decision to my publisher.
Video note: when I mention my own books begin on pages 6-7, by the way, it’s because they are 40 pages long (I forgot to mention this, sorry).
For pre-published picture book writers with no track record, I recommend sticking to the standard 32 page template; you can always discuss the possibility of increasing the page count after your mss has been acquired.
Some resources you might find useful:
Creating Picture Books: my collection of how-to guides, free templates and resource lists.
My free, print-ready archive: includes more templates.
Tara Lazar’s post on Picture Book Dummies, Picture Book Construction, and Knowing Your Layout