Because people who want such things have not created them yet.Why there is not anything like these for FreeBSD?
Because books are more expressive than pretty little pictures.
There's this free alternative... MIT OpenCourseware: Operating System Engineering. It's not FreeBSD specific, but it's of UNIX.
I don't see a problem with buying a book. It's a free operating system, and the authors put their time and knowledge together to make their information available for a price. The knowledge doesn't expire, so it's not a bottleneck restriction. Instead of paying for a proprietary OS, the cost goes into an interesting book.